Avian leukosis virus subgroup J (ALV-J) is the newest member of the avian oncogenic retroviruses. After the first isolation of the ALV-J prototype virus, HPRS-103, more than 10 years ago in the United Kingdom (21), viruses belonging to this subgroup have spread rapidly to many countries, becoming one of the major pathogens facing the broiler meat industry worldwide (26). The env gene of ALV-J is closely related to that of a novel group of chicken endogenous retroviral elements designated EAV-HP (24), suggesting that ALV-J has emerged by genetic recombination (17). Compared to the pathogenic ALV subgroups, such as A and B, which primarily induce lymphoid leukosis in genetically susceptible birds (18), ALV-J isolates predominantly induce myeloid leukosis (ML), a property thought to be associated with their tropism for the cells of the myeloid lineage (1). Previous studies have shown that HPRS-103 and other ALV-J isolates do not transform chicken bone marrow cell cultures in vitro and that the tumors induced by these viruses occur after long latent periods (19). These observations and the demonstration that the nucleotide sequence of the viral genome does not carry any viral oncogenes (2, 3) suggested that ALV-J-induced oncogenesis occurs by the activation of oncogenes through the mechanism of insertional mutagenesis (13).Although the tumors induced by HPRS-103 are of late onset, occurring at a median age of 20 weeks (19), we have previously shown that acutely transforming ALVs that induce rapid-onset tumors could be isolated from about 60% of lateonset tumors (20). Many tumors obtained from field cases of ML also contained acutely transforming viruses, suggesting that generation of acutely transforming ALVs is a common feature of ALV-J-induced oncogenesis. Most of these virus isolates were able to transform chicken bone marrow or monocyte cell cultures in vitro and induce rapid-onset tumors when inoculated into susceptible birds, a property attributed to the transduction of oncogenes. The acutely transforming ALV-J strain 966 was recovered from a myeloid tumor induced experimentally by . This virus transformed peripheral blood monocyte and bone marrow cells and induced rapid-onset tumors in chickens (20) and turkeys (28). Peripheral blood monocytes and bone marrow cells from different lines of chickens showed variation in the relative susceptibility to transformation by ALV-J strain 966 (1). This variation was correlated with the relative susceptibility to the induction of ML by HPRS-103, suggesting the involvement of common cell-specific viral and/or host factors in oncogenesis induced by these two viruses. In order to identify the viral genes and oncogenes that are involved in the rapid induction of tumors, we have determined the complete sequence of the proviral genome of ALV-J strain 966. In this paper, we demonstrate the genome structure of the provirus of the 966 strain of ALV-J and compare its sequence with that of HPRS-103 and other acutely transforming avian retroviruses. We also present data demonstratin...