We have previously described several helper independent vector constructions (S. Hughes and E. Kosik, Virology 136:89-99, 1984; J. Sorge and S. H. Hughes, J. Mol. Appl. Genet. 1:547-599, 1982; J. Sorge, B. Ricci, and S. Hughes, J. Virol. 48:667-675, 1983), all of which derive from Rous sarcoma virus. In this report we describe three improvements in the earlier constructions. First, the vectors have been restructured as proviruses, which considerably improves the efficiency of virus production following acute transfection. Second, a series of miniplasmids have been developed, which we call adaptors, and these miniplasmids can be used to convert virtually any DNA segment into a ClaI fragment suitable for insertion into the retroviral (or other) vectors. Adaptors have been developed that supply regions of functional significance, including a splice acceptor and an initiator ATG. Finally, the region of env defining subgroup specificity, A in the original vectors, has been substituted by the corresponding regions of subgroup B and D viruses, giving vectors with additional subgroup specificities and increased host ranges.