Participants in ATN 024 are HIV-infected youths aged 12 to 24 years, while participants in ATN 025 are HIV-uninfected youths aged 12 to17 years, thus participants in this substudy will be HIV-infected and uninfected youth aged 12 to 24 years. All eligible youths must be negative for HBV core antibody, HBV surface antigen, and HBV surface antibody at the time of enrollment in to the parent protocols.