The clinical study AVX-301 studies the efficacy and safety of 800mg and 1200mg BID ATC in combination with an optimized background in patients who are HIV-1 infected and have failed treatment with emtricitabine or lamivudine and have confirmed M184V/I mutation. Patients who do not achieve at least a 0.5 log10 copies/mL decrease in plasma HIV RNA by Week 16, or who experience a >1.0 log10 copies/mL increase (confirmed on two separate occasions at least one week apart) in plasma HIV RNA from nadir at or after Week 16, are eligible for screening for entry into this study. In addition, patients who complete study AVX-301 are also eligible for screening. The clinical study AVX-302 is similar and studies the efficacy and safety of a single dose of ATC (determined in AVX-301). Patients in AVX-302 who experience virologic failure or lack of virologic response will similarly be eligible for screening for entry into this study. In addition, patients who complete study AVX-302 are also eligible for screening. This is a phase 3, open-label, multi-center study of up to 96 weeks duration. Patients will receive for up to 96 weeks open-label ATC, initially at a dose of 1200mg orally BID. Following selection of the optimum ATC dose in protocol AVX-301 and termination of one of the two ATC doses (800mg or 1200mg orally BID) in protocol AVX-301, patients in this protocol will be transferred to the optimum dose, if required. The total number of patients expected to be enrolled in studies AVX-301 and AVX-302 is approximately 1866 in total.Patients will receive ATC orally in combination with their existing ART. Background antiretroviral therapy (ART) may be revised at any time during the study at the discretion of the investigator but must not include lamivudine, emtricitabine, zalcitabine or any other deoxycytidine analogue NRTI. The study will examine the safety and tolerability of long term treatment with apricitabine in combination with other ART.