The major emphasis of this section's program during the past year has been the application of recombinant DNA technology to construct molecular clones B- and C-type retroviruses. These reagents will be of invaluable assistance in the analysis of the mechanisms of regulation of viral expression and of viral tumorigenesis. Recombinant clones have been generated for the unintegrated circular intermediates of HaSV, KiSV, and FrMuLV, and for the integrated copies of AKR MuLV (both endogenously and exogenously acquired) and MMTV viruses. The HaSV, FrMuLV and AKR recombinant molecules have been shown to be biologically active in transfection assays, and molecular analysis of three unique HaSV circular clones revealed the presence of molecules containing non-duplicated, tandem-duplicated and tandem-triplicated regions with respect to the redundant sequences present at the ends of the in vivo linear intermediates. Analysis of the integrated AKR and MTV clones demonstrates that the integrated viral sequences are co-linear and non-permuted compared to the RNA genome, but that independent viral recombinants contain different sets of adjacent cellular sequences.