Members of the ATP binding cassette transporter family are responsible for the cellular efflux of a broad range of endogenous compounds and xenobiotics in multiple tissues. The ABCG2 (ABCP, BCRP, MXR) protein, which is highly expressed in the gastrointestinal tract and liver as well as numerous multi-drug resistant cancer cells, is considered to be of particular importance in governing drug absorption, elimination and cellular sensitivity to a wide variety of clini-cally important drugs. A wealth of recent literature has also indicated that inhibition of this transporter may result in drug-drug interactions. Furthermore, genetic polymorphisms in the gene encoding ABCG2 have been described, which can sig-nificantly affect expression, cellular localization, and/or substrate recognition in vitro and in vivo. Alteration of ABCG2 transporter function by either of these mechanisms has been demonstrated to contribute to interindividual variability in drug disposition and treatment outcome (toxicity or respo nse) with certain, but not all, substrates for ABCG2. In this re-port, we provide an update on this rapidly emerging field.