Myeloblastosis-associated viruses (MAV) are replication competent avian retroviruses responsible for the induction of lymphoid leukosis, osteopetrosis, and nephroblastoma. Although both the route of infection and the strain of host used has been reported to be a critical factor in determining the outcome of viral infection, genetically distinct strains of MAV that exhibit a multiple pathogenic potential have been molcularly cloned. Osteopetrosis is a proliferative disease of the bones and nephroblastoma is a kidney cancer. Both diseases occur in chickens a few weeks after MAV injection. In both cases, the nature of the target cells and mechanisms of transformation induced by MAV remain to be established. Molecular cloning and sequencing of three MAV proviral genomes inducing both osteopetrosis and nephroblastoma or only nephroblastoma have allowed the identification of viral determinants essential for osteopetrosis induction. For the last decade we have focused our attention on the MAV-induced nephroblastoma because it is a unique animal model of the human Wilms' tumor. Studies that we have conducted to understand the molecular basis of MAV tumorigenic potential have led to the identification of viral sequences required for tumor induction and to the discovery of a new cellular gene (nov) likely to play a critical role in avian and human nephroblastoma development.