This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A5202 is a phase IIIB, randomized, four-arm study, comparing the efficacy, safety and tolerability of open-label ritonavir (RTV)-enhanced atazanavir (ATV) to efavirenz (EFV), in combination with either daily emtricitabine (FTC)/tenofovir (TDF) or abacavir (ABC)/lamivudine (3TC) and of partially blinded ABC/3TC compared to FTC/TDF in combination with either RTV-enhanced ATV or EFV as initial therapy for HIV-1 infection. The three primary objectives are: (1) To compare virologic efficacy between regimens with virologic failure defined as the time to confirmed plasma HIV-1 RNA level >1000 copies/ml or after 16 weeks and before 24 weeks or >200 copies/mL at or after week 24. (2) To compare the safety between regimens with safety defined as the time to first development of Grade 3 or 4 sign, symptom, or laboratory abnormality that is at least one grade higher than at baseline. (3) To compare the tolerability between regimens with tolerability defined as the time to change in one or more drugs in the initial treatment regimen. A5224s will explore the metablic changes in HIV-1 infected subjects receiving either FTC/TDF or ABC/3TC with either EFV or ATV with RTV. The primary objects of A5224s are to assess changes within treatment arms in peripheral fat changes (limb fat) after the initiation of an ARV regimen containing ABC/3TC or TFC/TDF (Arms B/D or A/C) and to compare the effects of initiation of an ARV regimen containing ABC/3TC with those of a regimen containing FTC/TDF on bone mineral density, as assessed by lumbar spine and hip dual energy x-ray absorptiometry (Arms B/D versus A/C). The hypotheses are: (1) FTC/TDF and ABC/3TC arms in Af5202 will be associated with relatively little loss of limb fat; and (2) TDF-containing arms will lead to higher rates of bone loss when compared to the other NRTI arms.