Abstract
Three genetically distinct types of chicken sarcoma virus Bratislava 77 (B77 virus) differing in their ability to infect duck cells were identified. B77 virus type I does not infect duck cells; B77 virus type II has a low efficiency of infection of duck cells; and B77 virus type III has a high efficiency of infection of duck cells. B77 viruses type I and III are produced by spontaneous mutation during the growth of B77 virus type II in chicken cells. The spontaneous mutation of B77 virus type II TO B77 virus type III occurs with a high rate (approximately 1 mutation per 50 infected cell generations), requires cell replication, and neither occurs during the synthesis of viral DNA on an RNA template nor during the transcription of progeny viral RNA from the provirus. The rate of spontaneous mutation of B77 virus type II to B77 virus type I is greater than the rate of spontaneous mutation of B77 virus type II to B77 virus type III.