The genomic organization of HIV-1 and the family of HIV-2 and SIV viruses1-4 is similar. However, there is an open reading frame, orf-x, that is present in HIV-2 and SIV, but not in HIV-13,4. The extent of protein sequence conservation in orf-x between HIV-2ROD and SIVMAC4 suggests that this open reading frame encodes a gene that may be important for infectivity or replication. Here, we show that the orf-x products of SIVMAC and HIV-2SBI-6669 are virion-associated and that the introduction of a premature stop codon into orf-x, did not abrogate virus infectivity and replication in vitro. Antibody reactivity to the orf-x product was detected in 35 of 42 HIV-2 positive serum samples and 11 of 52 SIV seropositive monkeys. No such antibodies were detected in HIV-1 positive donors, blood donors seronegative for both HIV-2 and HIV-1, or SIV seronegative monkeys. This suggests that orf-x is dispensable for in vitro replication of SIVMAC and that the orf-x gene product of HIV-2 or its antibody can be used to distinguish HIV-2 from HIV-1 infection.
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