Inclusion Criteria:

• Age > 18 year to < 25 years.
• Confirmed diagnosis of HIV-1 infection defined as one positive assay supported by documentation from the subject’s medical record. The result may be any of the following:
• HIV-1 DNA PCR,
• HIV-1 RNA PCR (> 5,000 copies/ml),
• Standard ELISA with confirmatory western blot performed after 18 months of age, or
• HIV culture.
• CD4 cell count: no restrictions.
• Viral load: no restrictions.
• Current treatment with stable antiretroviral combination therapy with at least 3 active drugs for a minimum of 28 days. The treatment regimen will not be started or changed for the purposes of participation in this study. Rather, this study will measure kinetics of the drugs in patients who have been receiving therapy at the direction of their treating physician.
• Regimen must be prescribed at FDA-approved doses for age.
• Regimens allowed:
• Atazanavir 300 mg po once daily plus ritonavir 100 mg po once daily, and
• Tenofovir 300 mg po once daily, plus
• At least one other antiretroviral medication prescribed at FDA-approved dose for age, excluding other protease inhibitors and NNRTIs.
• Ability and willingness to be contacted by study personnel daily for the two days prior to the pharmacokinetics visit, to take antiretroviral medicines at the same time in the morning daily for at least 3 days (one of those days being the day of the PK study visit), and ability and willingness to return to the clinic the day after the observed administered dose for a follow-up measurement of plasma drug concentration.
• Ability and willingness to provide written informed consent.

Exclusion Criteria:

• Pregnancy.
• Active therapy for malignancy.
• Known presence of gastrointestinal disease that would interfere with drug administration or absorption.
• Grade 3 or higher ALT or AST.
• Grade 3 or higher Creatinine.
• Concurrent treatment with another protease inhibitor or a non-nucleoside analogue reverse transcriptase inhibitor.
• No evidence of anemia greater than Grade 1 according to the ATN Toxicity Table for Grading Severity of Adolescent Adverse Experiences (see Chapter 11 of ATN MOGO).