Thirty years have passed since Gross (1951) first reported that the heritable lymphoma of AKR mice could be transmitted to other mice by a virus. Subsequent studies on lymphoma in X-irradiated C3H mice (Gross 1959) and C57 BL mice (Lieberman and Kaplan 1959) led Latarjet and Duplan (1962) to suggest that the activation of a latent virus in radiation leukemogenesis was akin to induction of lysogenic bacteriophage. While Temin’s DNA provirus hypothesis (1964) provided a molecular model for the chromosomal inheritance of retrovirus genomes, this concept was not accepted by most virologists until the discovery of reverse transcriptase in 1970. By then convincing data had also accrued to reveal that retrovirus genes existed as host genetic traits in mice and chickens.