1. Molecular organization of Moloney sarcoma virus was studied by nucleotide sequence analysis. The entire transforming region including the terminal redundancies of this viral genome were sequenced. These studies revealed several important biochemical features of this viral genome which enables us to understand the transcription, translation and mechanisms of integration of this viral genome. 2. Abelson-MuLV genome in its integrated form was cloned in E. coli. Biochemical analysis is currently being carried out to understand the structural organization of this viral genome. 3. Studies were carried out to understand cellular targets for in vivo transformation by two clonal replication competent type C viruses--Moloney and Rauscher MuLV. Moloney MuLV was found to induce T-cell tumors while Rauscher MuLV produced tumors of B-cell origin. This provides a model system to understand the mechanisms which are responsible for tissue specificity of these two viruses.