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Most replication-competent retroviruses cause cancer only after a long latent period, by insertional mutagenesis of the host genome, usually resulting in activation of a cellular proto-oncogene. In contrast, acute-transforming retroviruses, which have transduced host proto-oncogenes, cause rapid tumor formation and death. In both cases, some of the same genes are overexpressed and/or mutated, including genes involved in mitogenic signaling, cell cycle control, and cell survival. We are near the 100th anniversaries of the discoveries of both leukemia (1908) and sarcoma (1910) viruses in birds. In honor of this important milestone, this review will be focused mainly on studies of oncogensis by avian viruses, which paved the way for studies with many other oncogenic retroviruses. Rous sarcoma virus (RSV) causes rapid oncogenesis by high level expression of an activated src gene tyrosine kinase. The host src gene was transduced by the virus without its C-terminal negative regulatory domain. In contrast, avian leukosis virus (ALV) induces B-cell lymphomas by insertional mutagenesis after the provirus integrates into the host genome. In addition to activation of classical proto-oncogenes, proviral insertions can activate cellular micro-RNAs called oncomiRs. The precursor of miR-155, which is upregulated in many human tumors, was first identified as a common ALV B-cell integration cluster (bic) in metastatic, long-latency lymphomas. Targets of miR-155 repression include tumor-suppressor genes, providing a novel mechanism for their inactivation in ALV-induced tumors. Telomerase reverse transcriptase is also activated by enhancer insertion in many ALV-induced lymphomas, providing a good model system for study of telomerase-dependent tumors. [ABSTRACT FROM AUTHOR]