Observational Study of HIV-Infected Subjects With X4-Tropic or Non-Phenotypeable Virus

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.