Livestock Production Science 74 (2002) 255 268
www.elsevier.com/locate/livprodsci
Transgenesis to improve animal production
Louis-Marie Houdebine*
Biologie du Développement and Biotechnologies, Institut National de la Recherche Agronomique, 78352 Jouy-en-Josas Cedex, France
Abstract
The improvement of animal production by transgenesis is expected to be complementary to other breeding techniques
including genetic selection (by conventional methods and using genetic markers), feed optimization, control of reproduction
and struggle against diseases. It may introduce new and efficient methods to solve various problems as is already the case for
plants. The success of transgenesis for animal production has been limited for 15 years by the difficulty and the cost to
generate transgenic farm animals. Gene addition can be achieved by DNA microinjection into one-cell embryos in rabbit and
pig. In ruminants, gene addition can be achieved by classical transfection in fetal cells further used to generate cloned
transgenic animals. The cloning techniques have been used successfully to replace gene by homologous recombination in
sheep and can be extended to goat, cow, pig and expectedly in rabbit. This may lead to precise gene inactivation or allele
replacement. In birds, several techniques can be used to generate transgenics. None of these techniques can presently be used
routinely to improve breeding. Transgenesis in farm animals is expected to protect animals against diseases, to reduce
rejection of pollutants, to optimize digestion, to improve growth and fertility, to optimize meat and milk composition, etc . . .
The production of pharmaceutical proteins in transgenic animals and the use of pig organs for xenografting are not expected
to have any significant direct impact on animal production. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Transgenesis; Farm animals
1. Introduction production traditionally had the strongest impact on
the improvement of animal production. These two
Improvement of animal production started with the areas have always been closely linked and this
beginning of breeding centuries ago. Four major remains true with the emergence of transgenesis.
complementary approaches have allowed consider- The conventional animal selection strictly relies on
able progress mainly during the last century: selec- the Darwin and Mendel rules which govern evolution
tion, reproduction struggle, against diseases and and transmission of the genetic properties of living
nutrition. Progress is still awaited in these different organisms. Mutations in genomes are randomly
fields and each of them will benefit from the recent generated during reproduction by errors in DNA
discovery in molecular genetics. Selection and re- replication, chemical DNA modifications and chro-
mosome rearrangement. In nature, the environment
favors the emergence of the best-adapted individuals.
*
Tel.: [phone]; fax: [phone].
In breeding, the retained genetic properties are those
E-mail address: [e-mail] (L.-M.
Houdebine). chosen by selection. The efficiency of selection is
0301-6226/02/$  see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0301-6226(02)00018-0
256 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
therefore highly dependent on mutations which observation of the animals can lead to the establish-
cannot be controlled and on the capacity to evaluate ment of lines having new genetic properties. In the
relevant genetic traits. Most of the biological func- best conditions, the systematic use of genetic
tions are governed by several genes often not located markers allow the identification of the genes respon-
on the same chromosome. The coordinated selection sible for the observed phenotypic property (Brown
of the involved genes is therefore complex. and Balling, 2001). The success of this approach is
It has been suggested that in microorganisms, possible only in species having a short reproduction
various stresses induce a destabilization of DNA. cycle and it cannot be applied to farm animals.
This mechanism jeopardizes the life of individuals Systematic genome mapping and sequencing will
but it increases the frequency of mutations and thus give access to all the genes of several farm animals.
the chance of the species to become adapted to new This will give the possibility to select animals not
environmental conditions and to survive. It is not only by the evaluation of their phenotypic properties
known if this mechanism also acts in animals. It but by the sequencing of the various alleles respon-
cannot therefore be used to improve animal selec- sible for the observed genetic traits (Fig. 1). An
tion. example can illustrate this point. It is well known
Mutations can be induced by chemical agents that milk quality is dependent on the expressed
administered in limited amounts to animals. Further alleles of milk protein genes. A systematic sequenc-
Fig. 1. The major uses of isolated genes. Genes can be isolated by classical cloning methods (1), by the use of microsatellite markers (2), or
by systematic sequencing of cDNAs and genomes (3). Isolated genes can be studied for themselves, used for genetic diagnosis and selection,
to produce recombinant proteins, for gene therapy or for transgenesis.
L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268 257
ing of their major alleles has been carried out. This attractive method to improve animal production.
information has rapidly been used to select sires However, transgenesis also suffers from several
harboring the best alleles. This technique is perfectly theoretical limitations.
precise and can be applied at any period of the The presence of a foreign gene may have a
animal life. negligible impact on a biological function which is
Until the farm animals genome is better described, controlled by multiple genes. Yet, it should not be
selection will maintain a degree of imprecision. In believed that only monogenic traits can be improved
most cases, animals are still selected by evaluation of by transgenesis. Indeed, the genes involved in the
the phenotypic properties. This means that the select- expression of a biological function often act in
ed genes are unknown. The rearrangement of chro- cascade and the overexpression of only one of them
mosomes during gamete formation allows the gene may modify quite significantly this function. A
of interest to be selected even if it is unknown. It historical experiment illustrates perfectly this point.
also coselects many other unknown genes, which Growth in animals is obviously dependent on many
may have deleterious effects for the life of the genes. Yet, the overexpression of growth hormone
animal (Fig. 2). Many examples can illustrate this transgene is sufficient to enhance or accelerate the
point. development of mice and fish.
Even when all the farm animal genes are iden- It is now admitted that the complexity of living
tified, selection will remain strictly dependent on the organisms is not simply related to gene number. It is
random mutations occurring during reproduction. indeed striking that mammals do not have many
Transgenesis suppresses a number of these problems. more genes than plants. Accumulated experimental
In the optimized situations, the isolated gene trans- data support the idea that the interactions between
ferred to the animal is well-known and its biological genes and gene products which are more numerous
effects are predictable to a large extent. On the other in animals can account for their complexity (Szat-
hand, gene transfer induces only a single alteration hmary et al., 2001). This implies that a gene may
of the host genome (Fig. 2). have several functions according to the cells in
For these reasons, transgenesis appears to be an which it is expressed and to the period of the animal
Fig. 2. The comparison of standard selection and transgenesis. Standard selection relies essentially on the identification and preferential
reproduction of individuals having a natural mutation in one or several genes. The transfer of the mutation to progeny is accompanied by the
transfer of other regions of the chromosome bearing the mutation. This often leads to deleterious side-effects. Transgenesis is a more precise
technique of selection.
258 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
life. The addition of a foreign gene to a genome can variations of transgene expression which are some-
therefore generate unpredictable interactions with times observed in the individuals of a transgenic line.
some cellular components and thus modify pheno- Despite these limitations, transgenesis is consid-
typic properties of the animals in a more or less ered by biologists as a more precise method than
unexpected manner. conventional selection. A major point should also be
Epigenetic mechanisms are known to exert a kept in mind. Transgenesis is a potent and rapid way
control on gene expression. These mechanisms es- to enhance biodiversity by providing animals with
cape heritability and thus selection. Some of these foreign genetic information, an event which has no
mechanisms are known. Most of the retrotransposons chance of occurring in conventional breeding.
and other selfish DNA sequences are methylated and
thus inactivated. During early embryo development,
DNA is transiently demethylated and selectively 2. The methods to generate transgenic animals
remethylated. For a part of selfish DNA, remethyla-
tion occurs randomly. These sequences are therefore Transgenesis includes fundamentally two different
active or not in an unpredictable manner. It is kinds of experiments: gene addition (Fig. 3) and
admitted that genes located in the vicinity of a gene replacement by homologous recombination
retrotransposon may be under the control of the (Fig. 4). In the first case, additional genetic in-
regulatory elements of the retrotransposon. The formation is introduced stably into the genome of the
expression of a given gene is therefore dependent on animal. The foreign gene generally codes for a
the random methylation of the neighbor retrotran- protein which belongs to the same species or to
sposons. The same is true for transgenes (Whitelaw another species. The additional gene may have been
and Martin, 2001; Symer and Bender, 2001; Rakyan transferred to inhibit the expression of an endogen-
et al., 2001). These phenomena can explain the ous gene. In the second case, the foreign DNA may
Fig. 3. The mechanisms leading to the random integration of a foreign gene into an animal genome. The foreign DNA sequences recognize
short and partially homologous regions of the genome. Reparation mechanisms integrate the foreign DNA. Before integration, a homologous
recombination mechanism generated polymers of the foreign gene organized in tandem.
L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268 259
Fig. 4. The experimental protocol leading to specific gene replacement. A gene construct containing two long regions strictly homologous to
the targeted host gene and containing a foreign DNA region transferred to cells. The homologous sequences recombine and the targeted gene
is replaced by the foreign gene. The cells in which gene replacement occurred are saved by double selection not shown here.
be an inactive gene. The endogenous gene thus and monkey oocytes (Chan et al., 1998, 2001).
becomes knocked out. The foreign DNA may be a Retrotransposon vectors are being used to generate
mutated but active version of the endogenous gene or transgenic Drosophila (Kayser, 1997), medaka (Hac-
a quite different gene. kett et al., 1997), chicken (Shermann et al., 1998),
silk worms (Tamura et al., 1999) and several other
2.1. Gene addition invertebrates.
Gene transfer in isolated spermatozoa followed by
The original method defined to generate transgenic fertilization can generate transgenic mammals, chick-
mice consists of microinjecting the isolated gene into en and fish (Baccetti and Spadafora, 2000). A
the promocleus of one cell embryos. This method permeabilization of sperm membrane followed by
has been extended to several other mammals (rat, fertilization by intra cytoplasmic sperm injection
rabbit, pig, sheep, goat and cow). In bovine, the (ICSI) is an efficient and the only way to generate
generation of one cell embryo must be performed by transgenic xenopus (Marsh-Armstrong et al., 1999).
in vitro oocyte maturation and fertilization (Krim- Although attractive, gene transfer via sperm incu-
penfort et al., 1989). The DNA injected into nucleus bated with DNA remains a poorly efficient method
is recombined to form concatemers organized in for most species since the yield of transgenesis is
tandem (Fig. 3). low and foreign DNA is often heavily rearranged and
In lower vertebrates and invertebrates, pronuclei inactivated. A recent work demonstrated that sper-
are not visible and DNA must be injected into matozoa incubated with monoclonal antibodies rec-
cytoplasm. Concatemers organized in head to tail ognizing a specific antigen and bound to DNA
and tandem are then formed before migration to the transfer gene with high efficiency to oocytes. Fertili-
nucleus and integration. zation was carried out in vitro in mouse, by artificial
DNA injection into cytoplasm is followed by a insemination in chicken and by injection into uterine
quite variable success of integration according to horn in pig. In all cases, up to 30% of the born
species. The use of retroviral or retrotransposon animals were transgenic. The transgenes were ex-
vectors are then required. Retroviral vectors have pressed and transferred to progeny. Interestingly, the
been designed to transfer foreign genes into chicken same monoclonal antibody recognized a sperm an-
embryonic cells (Ronfort et al., 1997) or into cow tigen in several mammals, including human, and in
260 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
lower vertebrates. This method might greatly sim- to progeny. Embryonic stem cells (ES) are currently
plify gene addition in animals (Qian et al., 2001a,b) used in mouse for this purpose since 1989 (Capecchi,
1989) and it allows the knock out of about 5000
2.2. Gene replacement genes (Fig. 5). For unknown reasons, ES cells lines
have not been established so far in other species and
Gene replacement relies on homologous recombi- gene replacement by this method remains restricted
nation (Fig. 4). This is a rare event and appropriate to two lines of mouse. Interestingly however, specific
vectors must be used to clone cells into which gene vectors have been defined recently to replace genes
replacement occurred (Viville, 1997). One of the by homologous recombination in Drosophila (Ber-
properties of these vectors is to allow selection of the nards and Hariharan, 2001). Recent data also re-
cells in which homologous recombination occurred. vealed that pluripotent cells capable of forming
Cells in which gene replacement took place must chimerae and transfer their genome to progeny were
be able to participate in the development of an identified in chicken (Petitte et al., 2001) and fish
embryo followed by the transmission of the mutation (Collodi and Fan, 2001)
Fig. 5. The transmission of a mutation by the generation of chimeric animals. The multipotent embryonic cells in which gene replacement
occurred are transferred into a recipient embryo and participate in its development. The mutation can be transmitted to progeny.
L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268 261
The conditions defined to clone sheep by transfer cow (V. Pursell, personal communication) and pig
of nuclei from somatic cells of adults into enucleated (PPL Therapeutics, unpublished data) (Fig. 6). Inter-
oocytes, have been used to add genes to sheep estingly, the same method allowed gene replacement
(Schnieke et al., 1997), goat (Baguisi et al., 1999), in sheep (Ayares, 1999; McCreath et al., 2000) and
Fig. 6. The transmission of a mutation using the cloning technique. The fetal cells in which gene addition or replacement occurred are used
to generate living embryos after transfer into enucleated oocytes. The mutation is transmitted to progeny.
262 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
mouse (Readeout et al., 2000) (Fig. 5). The use of transcription or translation is one possibility (Up-
the method is not an easy task for several reasons: (i) egui-Gonzalez et al., 2000). Single or double strand
the primary fS
tal fibroblasts presently used for gene RNA inducing an RNA interference (Bass, 2001;
replacement have a limited capacity to divide; (ii) Elbashir et al., 2001) are also good candidates to
homologous recombination is generally less frequent prevent expression of a cellular or viral gene.
in somatic cells than in ES cells; and (iii) cloning by Ribozymes used in optimal conditions can also
nuclear transfer has a low yield which is further degrade specifically a given mRNA (Warashina et
diminished when cells are previously cultured al., 2001).
(Wells, 2001; Denning et al., 2001). Transdominant negative proteins overexpressed
Despite the difficulties presently encountered it from a transgene and acting as decoys can also
seems reasonable to think that these methods are efficiently and reversibly annihilate the action of a
being improved and applied to species other than cellular protein.
ruminants.
3.3. The conditional expression of transgenes
3. The vectors for transgene expression Several systems implementing particular vectors
are capable of controlling transgene expression by
3.1. The vectors for a reliable expression specific inducers not acting on endogenous genes
(Rivera et al., 1996; No et al., 1996; Wang et al.,
The first transgenic mice obtained by DNA mi- 1997; Rossi et al., 1998; Forster et al., 1999; Blau
croinjection did not express efficiently their trans- and Rossi, 1999; Fussenegger et al., 2000). The
gene. An explanation of this unexpected result was system based on tetracycline and derivatives as
given about 10 years later. inducers is presently the most frequently used sys-
Gene constructs must contain at least a promoter, tem.
a naturally transcribed region or a cDNA and a Homologous recombination can be specifically
transcription terminator. At least one intron should induced by the Cre-Lox P (Wunderlich et al., 2001)
be added to cDNA (Palmiter et al., 1991) and or the Flp-FRT systems (Umana et al., 2001) previ-
preferably a gene insulator. Gene insulators are well- ously integrated LoxP or FRT sequences by the
defined sequences in some cases (Taboit-Dameron et specific Cre and Flp recombinases, respectively.
al., 1999) or contained in long genomic DNA These systems allow the conditional elimination of
fragments (Giraldo and Montoliu, 2001). Insulators DNA sequences bordered by LoxP or FRT sequences
prevent silencing of transgenes by the neighbor and even chromosome rearrangements (Herault et
chromatin (Bell et al., 2001). al., 1998). They are also efficient tools to introduce a
Other recommendations to avoid unreliable ex- foreign DNA sequence at a defined site of a genome
pression of transgenes have been reported elsewhere where LoxP has been previously integrated (Araki et
(Houdebine et al., 2002). al., 1997; Feng et al., 1999; Schubeler et al., 2000).
3.2. The vectors to inhibit endogenous gene
expression 4. The applications of animal transgenesis
Gene replacement is an efficient but laborious Thousands of transgenic mice Drosophila,
method to inhibit the expression of a given endogen- Caenorhabditis elegans and medaka have been
ous gene. Moreover, it is generally performed at the generated to study gene function and regulation. This
early stages of development and it is irreversible. practice is still being amplified by the availability of
Transgenes capable of inhibiting specifically and all the genes from several genomes.
irreversibly an endogenous or a viral gene are highly Human diseases can often be mimicked in mice by
desirable. Transgenes coding for RNA forming a gene addition or replacement (Berns, 2001; Johnson
triple helix with DNA or mRNA and which inhibit et al., 2001). These animals which may be species
L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268 263
other than mice, such as rabbits (Fan et al., 1999) are physiology of the animals. Only a few studies have
sometimes also precious tools to evaluate the effects been performed so far in this direction. It is interest-
of new pharmaceuticals. ing to mention the existence of transgenic mice
Gene transfer is an essential way to obtain recom- expressing the bacterial lysostaphin gene which
binant proteins for pharmaceutical use from milk or prevent mammary infection by Staphylococcus au-
other biological fluids (Houdebine, 2000). reus (Kerr et al., 2001). This study is being extended
Transgenesis appears to be the mandatory ap- to cows.
proach to study the rejection mechanisms of xeno- Other transgenes may contribute to optimize feed
grafts and to prepare pig organs and cells for patients digestion. A case in point of this is transgenic mice
(Houdebine and Weill, 1999). expressing a bacterial phytase gene in their saliva
Transgenesis implementation for improvement of (Ward, 2001; Golovan et al., 2001a) and transgenic
animal production is still in its infancy. Several pigs expressing also a phytase gene in their intestine
projects can be envisaged (Table 1) or are being (Golovan et al., 2001b). These animals which digest
developed (Table 2). Some transgenes may confer phytic acid present in plants release lower amounts
resistance to diseases to the animals. These genes of phosphate into the environment. This may dimin-
may be natural resistance genes or artificial genes ish pollution and the pigs can use phosphate which
described above which inhibit the expression of accelerates their growth.
essential genes of pathogens (Müller, 2000). This Other digestive enzymes can be brought by vari-
kind of transgenesis is not expected to modify the ous transgenes to improve feed digestibility. This
Table 1
The possible genes to improve animal production
Biological function Expected advantages
Resistance to diseases Lower use of antibiotics
Higher welfare
Simpler breeding
Higher production
Lower infection risk for humans
Digestion and metabolism Lower pollution
Higher production
Better feed use
Adaptation to available feed
Deep metabolic change
Milk composition Reduced allergenicity and intolerance
Optimized protein composition
Optimized lipid composition
Protection against disease
(active and passive immunization)
Enhanced nutriceutical content
Wool growth Enhanced wool growth
Optimized wool composition
Carcass growth Higher muscle development
Lower lipid storage
Better lipid composition
Lower feed composition
Reproduction Higher prolificity
264 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
Table 2
The transgenes under study to improve animal production
Genes Animal Tissues Observed effects
Lysostaphin Mouse Mammary gland Resistance to mastitis
Cow // ?
Lactoferrin Cow Mammary gland ?
Lysozyme Cow Mammary gland ?
PrP (KO) Sheep All tissues ?
Phytase Mouse Salivary gland Lower phosphate release
Pig // under study
Carbohydrate Rabbit Intestine Under study
digestion enzyme Fish // Under study
Acetate glucose Sheep Under study
Growth hormone Pig Several Enhanced growth
Sheep Several Enhanced growth
Fish Several Enhanced growth
IGF1 Pig Muscle Muscle growth
Myostatin dominant Mouse Muscle Muscle hypertrophy
negative
Lactase Mouse Mammary gland Lower lactose content
Anti a-lactalbumin Mouse Mammary gland No lactose, low milk
antisense production
Phe2 human Cow Mammary gland Phe2 protein for patients
a-Lactalbumin with phenylketoneurea
a-Lactalbumin Pig Mammary gland Improved piglet feeding
K-casein Mouse Mammary gland Lower micelle size
IgG Mouse Mammary gland Antiviral action
Serine cysteine Mouse Several tissues Enhanced hair growth
Sheep // death of embryos
Keratins Sheep Skin Modified wool
composition
may also theoretically reduce feed wastage, diminish pressed in most fish species. This genetic modi-
pollution and enhance animal growth. fication would allow fish developed in aquaculture
Deeper changes in animal metabolism may be facilities to be fed with cheap carbohydrate rich feed
envisaged. Several ambitious projects are underway. rather than with costly protein based-products.
One consists of transforming acetate (a major metab- Milk composition may be optimized more easily
olite produced by digestion in ruminants) into glu- by transgenesis than by selection (Houdebine, 1998;
cose. This is expected to improve the utilization of Pintado and Gutierrez-Adan, 1999). A reduction of
ruminant feed (Ward, 2000). Another project aims at lactose concentration has been obtained in mice by
providing fish with key genes required for carbohy- the secretion of lactase in milk (Jost et al., 1999).
drate digestion. These genes are very poorly ex- This may contribute to enhance tolerance of many
L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268 265
people to lactose, to lower the amount of water in Wool growth and composition can be altered in
milk and to reduce the frequency of mastitis sheep by transgenesis (Bawden et al., 1999).
(Whitelaw, 1999).
Beta-lactoglobulin is known to be one of the major
milk allergens. The knock out of this gene should 5. Conclusions and perspectives
solve this problem.
A modification of the casein proportion or of their Although several lines of transgenic animals could
variants in milk is theoretically possible by trans- be proposed to human consumers, none of them are
genesis. Allele replacement as gene addition may be presently on the market. This is the case for pigs and
appropriate to reach this goal. An overexpression of fish having enhanced growth. In both cases, tests are
aS1-casein (Chanat et al., 1999) or k-casein (Bösze being performed to evaluate the potential negative
et al., 2001) are expected to improve milk quality. impact on human health. In the case of fish, the
Milk appears to be one of the ideal vehicles for possible uncontrolled dissemination of the transgene
nutriceuticals, oral vaccines or protective immuno- and its transfer to wild animals (Muir and Howard,
globulins against pathogens of the digestive tract 1999) is a major hurdle for the use of these animals.
(Saif and Wheeler, 1998; Castilla et al., 1998; Kolb The generation of genetically modified animals for
et al., 2001). Studies to evaluate the feasibility of this human consumption has come slowly in comparison
approach are in course. It is interesting to mention to plants. This is obviously due to technical problems
that transgenic cows expressing in their milk a which are now almost solved. Indeed the possibility
human mutant of a-lactalbumin devoid of phenylala- to add randomly or in a targeted manner (Kolb et al.,
nine have been obtained. This protein purified from 1999) and to replace gene by homologous recombi-
milk is a good candidate to be the source of nation via the cloning technique has opened new
amminoacids for patients suffering from avenues for the improvement of animal production
phenylketonurea (PPL Therapeutics, unpublished by transgenesis.
data). The availability of all the genes of the major farm
Milk composition of farm animals may also be animals in the coming years will provide experimen-
optimized to favor development of offsprings. ters with good candidate genes for transgenesis. The
Bovine a-lactalbumin gene expressed in the milk of cost of transgenesis even in its improved versions
transgenic pigs improved the nutritional value of the and the time required to spread relevant transgenes in
milk and allowed more piglets to survive after herds will limit the application of transgenesis in
weaning (Bleck et al., 1998). The effects of the breeding. A candidate gene must be studied carefully
enrichment of pig milk by other transgenes are under to make sure that its transfer to farm animals is
study. economically justified (Bishop, 1998).
Growth, reproduction, carcass composition and Animal transgenesis has presently several advan-
other biological functions can theoretically be opti- tages over plant transgenesis. Allele replacement is
mized by transgenesis. Growth has been accelerated possible by homologous recombination in animals
or enhanced in various fish (Devlin, 1997) and pigs but not presently in plants. This offers the possibility
(Nottle et al., 1997) by the transfer of growth to perform more physiological gene transfer with
hormone genes. IGF1 transgene expressed specifical- reduced side-effects. On the other hand, essentially
ly in skeletal muscle increased muscle development aquatic animals and insects may potentially dissemi-
(Pursell et al., 2001). No side-effect of these trans- nate their genes into wild species whereas more
genes were observed. plants meet this problem.
A gene coding for a transdominant negative of Hence, the present technical state of the art
myostatin and expressed in muscle has been shown inclines to think that the era of transgenesis to
to enhance muscle development after birth in mice improve animal production started recently. New
(Wall, personal communication). These preliminary lines of farm animals generated by gene transfer will
results suggest that the process can be extended to be proposed in the coming years although at a
farm animals. relatively slow rate. Moreover, the acceptability of
266 L.-M. Houdebine / Livestock Production Science 74 (2002) 255  268
mice secreting virus-neutralizing antibodies in milk. Nature
these new foods by consumers will remain for some
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Chan, A.W., Chong, K.Y., Martinovich, C., Simerly, C., Schatten,
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Chan, A.W., Homan, E.J., Ballou, L.U., Burns, J.C., Bremel, R.D.,
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