Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) require coreceptors, in addition to CD4, to infect target cells. Some G protein-coupled receptors including CCR5, CXCR4, CCR3, and CCR2b in the chemokine receptor family have been identified as HIV coreceptors. An orphan G protein-coupled receptor was recently cloned and designated Bonzo, STRL33 and TYMSTR, and identified as HIV and SIV coreceptor. Bonzo/STRL33 serves as coreceptor for SIV, HIV-2 and HIV-1. The messenger RNA of Bonzo/STRL33 is expressed in lymphoid tissues and activated peripheral blood lymphocytes.