This family of transporters comprise OAT1, OAT2, OAT3, OAT4 and RST. UST1, UST3 and OAT5 are also considered as transporters of organic anions, however this has not yet been proven (Anzai et al., 2006). The recently identified urate transporter, URAT1 is very similar to RST in terms of amino acid sequence and tissue distribution. Additionally, URAT1 appears to be the human ortholog of murine RST and rat OAT1 (Eraly et al., 2004). Many drugs exist as organic anions at physiological pH levels and therefore OATs present must have a pivotal role in handling these compounds. Studies using Xenopus laevis oocytes showed that antiretroviral nucleoside drugs are substrates for OAT proteins despite the fact that they are not conventional organic anions (Strazielle et al., 2003).