This substudy will enroll volunteers from participants of ATN 024 and ATN 025. Participants in ATN 024 are HIV-infected youths aged 12-24 years while participants in ATN 025 are HIV-uninfected youths aged 12-17 years. These youths must also be negative for HBV core antibody, HBV surface antigen, and HBV surface antibody to be eligible.