The purpose of this study is to measure viral tropism over time in subjects with X4-tropic or non-phenotypeable virus while receiving standard of care therapy. This is an observational study. No investigational treatment will be administered through this study.

Inclusion Criteria:

• HIV-infected.
• Screening viral load at least 5000copies/mL.
• X4-tropic only or non-phenotypeable virus at screening.
• Total prior antiretroviral experience of at least 3 months and documented resistance to at least one drug in each of the following classes: NRTI, NNRTI, and PI, stable antiretroviral regimen (or no antiretroviral treatment) for at least 4 weeks before screening.
• Able to receive a ritonavir-boosted protease inhibitor during treatment studies.
• Women of childbearing potential must use specific forms of contraception.

Exclusion criteria:

• R5-tropic only or R5/X4-tropic virus at screening, changes to antiretroviral therapy from 4 weeks prior to screening until Day 1 of observational study.
• Pregnant or breastfeeding women.
• Significant ECG abnormalities or significant history of active pancreatitis, hepatitis, opportunistic infections, cancer, or severe illness.
• Current use of certain medications may exclude participation in this study.
• Additional qualifying criteria and laboratory test requirements to be determined by study physician.