We characterized HIV-1 RT variants either with or without the 3TC-resistant M184I mutation isolated from a single HIV-1 infected patient. First, unlike variants with wild type M184, M184I RT variants displayed significantly reduced DNA polymerase activity at low dNTP concentrations, which is indicative of reduced dNTP binding affinity. Second, the M184I variant displayed a ~10-13 fold reduction in dNTP binding affinity, compared to the M184 variant. However, the kpol values of these two RTs were similar. Third, unlike HIV-1 vectors with wild type RT, the HIV-1 vector harboring M184I RT failed to transduce cell types containing low dNTP concentrations, such as human macrophage, likely due to the reduced DNA polymerization activity of the M184I RT under low cellular dNTP concentration conditions. Finally, we compared the binary complex structures of wild type and M184I RTs. The Ile mutation at position 184 with a longer and more rigid -branched side chain, which was previously known to alter the RT-template interaction, also appears to deform the shape of the dNTP binding pocket. This can restrict ground state dNTP binding and lead to inefficient DNA synthesis particularly at low dNTP concentrations, ultimately contributing to viral replication failure in macrophage and instability in vivo of the M184I mutation.