The methionine-to-valine mutation in codon 184 (M184V) in reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) or simian immunodeficiency virus (SIV) confers resistance to (؊)-2-deoxy-3-thiacytidine (3TC; lamivudine) and increased sensitivity to 9-[2-(phosphonomethoxy)propyl]adenine (PMPA; tenofovir). We have used the SIV model to evaluate the effect of the M184V mutation on the emergence of resistance to the combination of 3TC plus PMPA. A site-directed mutant of SIVmac239 containing M184V (SIVmac239-184V) was used to select for resistance to both 3TC and PMPA by serial passage in the presence of increasing concentrations of both drugs. Under these selection conditions, the M184V mutation reverted in the majority of the selections. Variants resistant to both drugs were found to have the lysine-to-arginine mutation at codon 65 (K65R), which has previously been associated with resistance to PMPA in both SIV and HIV. Similarly, in rhesus macaques infected with SIVmac239-184V for 46 weeks and treated daily with The rapid emergence of drug-resistant mutants of human immunodeficiency virus (HIV) has proven to be a major obstacle in antiviral therapy for AIDS (7,32,36,63). Even highly active antiretroviral therapy has been limited by the emergence of multidrug-resistant HIV (32,46,61). This has led to efforts to identify drug combinations in which resistance to one drug results in a mutant virus that suppresses normal resistance to a second drug or in a mutant virus that is hypersensitive to other drugs (2,27,28,35,52).One mutation that results in phenotypic changes in HIV type 1 (HIV-1) that are useful in certain drug combinations is the methionine-to-valine mutation in codon 184 (M184V) of reverse transcriptase (RT). This mutation arises rapidly in therapy with the oxathiolane nucleosides, (Ϫ)-2Ј-deoxy-3Ј-thiacytidine (3TC; lamivudine) and (Ϫ)-2Ј-deoxy-5-fluoro-3Ј-thiacytidine [(Ϫ)-FTC], and results in high-level (Ͼ100-fold) resistance to these drugs (49,55). This is often preceded by emergence of a methionine-to-isoleucine mutation in codon 184 (M184I), which is quickly replaced by M184V (22, 50). This M184V mutation is located in the highly conserved YMDD motif of RT, which is directly involved in binding the incoming nucleotide during reverse transcription (19,20,24,53). It results in decreased processivity (1,3,21,23,51) and increased fidelity (11,41,62) of the DNA polymerase activity of RT in biochemical assays. However, the increase in fidelity was less than twofold in an M13 phage-based assay that scored the overall mutation rate after transfection of RT products into bacteria (9). M184V mutants of HIV-1 also have reduced replication rates in certain cell lines (1, 35, 51), and they have a broad array of changes in susceptibility to nucleoside analogs (15,28,35,54,55,64).A major effect of the M184V mutation is to suppress phenotypic resistance to 3Ј-azido-3Ј-deoxythymidine (AZT; zidovudine) when M184V is present along with mutations that normally confer resistance to AZT (28,55). Clinical...