Human retroviruses, such as HTLV-1 and HIV-1, encode accessory proteins, which regulate viral pathogenesis. The p12 protein of HTLV-1 is encoded from the pX-I open reading frame, and is critical for efficient virus replication in rabbits. Although dispensable for infection, replication, and immortalization of activated lymphocytes in culture, p12 expression is important for infection of quiescent lymphocytes. Similar to HTLV-1 p12, Nef is important for virus infectivity in SIV animal models. We questioned whether p12 could replace Nef in HIV-1, and reconstitute virus replication in culture. We found that p12 could complement for effects of Nef on HIV-1 infection of Magi-CCR5 cells or macrophages.