The invention provides isolated nucleic acids molecules, designated OAT5 nucleic acid molecules, which encode organic anion transporters. The invention also provides antisense nucleic acid molecules, recombinant expression vectors containing OAT5 nucleic acid molecules, host cells into which the expression vectors have been introduced, and non-human transgenic animals in which a OAT5 gene has been introduced or disrupted. The invention still further provides isolated OAT5 polypeptides, fusion polypeptides, antigenic peptides and anti-OAT5 antibodies. Diagnostic and therapeutic methods utilizing compositions of the invention are also provided.