The central effort of our laboratory for several years has been a detailed genetic analysis of the replication cycle of the Moloney murine leukemia virus (M-MuLV) and the human immunodeficiency virus type 1 (HIV-1). The major approach has been to create mutations in cloned DNA copies of the viral genomes and to determine the effect of the mutations on the viral life cycle after transfer of the altered DNAs into cells in culture. These genetic analyses have defined the functional domains of various viral proteins and the sites of their action on viral nucleic acids. We have also expressed reverse transcriptase and integrase in bacteria and studied these enzymes biochemically. We make use of the yeast two-hybrid system to monitor protein-protein interactions between viral proteins, and to identify new host proteins that interact with the Gag, Pol and Env gene products. Finally, we use genetic selections in mammalian cells to screen overexpression libraries and gene knock-down libraries to identify genes that restrict or are essential for virus replication, especially those affecting intracellular trafficking of the viruses. Our work also addresses the control of retroviral gene expression in embryonic stem cells. Recently we have begun charactering a retroviral element associated with a leukemia-like disease in the mollusk Mya arenaria, the soft-shell clam.