*Review* **Medically Important Alterations in Transport Function and Trafficking of ABCG2**

**László Homolya**

Research Centre for Natural Sciences, Institute of Enzymology, H-1117 Budapest, Hungary; homolya.laszlo@ttk.hu; Tel.: +36-1-382-6608

**Abstract:** Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in *ABCG2* may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in *ABCG2*, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

**Keywords:** ABC (ATP-binding cassette) transporters; multidrug resistance; transport; trafficking; urate; mutations; polymorphisms
