FIGURĘ 3-1 The major classes ot cancer-causing genes. Oncogenes act to accelerate cm during the G1 orgrowth phase ofthe cellcycle, whereas tumorsuppressor genes nomlly act as! breaks against celi growth and proliferation, with a check point just before DNA synthesls beglnsj In S phase. A third class ot repair genes fix DNA mismatches after synthesis and DNA replication. i