Pure Appl. Chem., Vol. 74, No. 11, pp. 2057–2081, 2002. © 2002 IUPAC 2057 ABCG subfamily of human ATP-binding cassette proteins* Stefan Lorkowski 1,‡ and Paul Cullen 1,2 1 Institut für Arterioskleroseforschung, Domagkstraße 3, 48149 Münster, Germany; 2 Ogham GmbH, Mendelstraße 11, 48149 Münster, Germany Abstract: ATP-binding cassette (ABC) proteins form one of the largest known protein fami- lies and have been found in all known organisms. Most members of the human ABC protein family are membrane-spanning transporters that use energy derived from the hydrolysis of ATP to transport specific substrates across cell membranes. Mutations in certain human ABC transporters of the subfamilies A, B, C, and D have been shown to cause a wide variety of inherited diseases such as the lung condition cystic fibrosis, the nervous degenerative condi- tion adrenoleukodystrophy (of Lorenzo’s Oil fame), hereditary macular degeneration of the eye (Stargardt’s disease), and inherited deficiency of circulating high-density lipoproteins (Tangier disease or familial hypoalphalipoproteinemia). Very recent studies showed that mutations in two members of the subfamily G of human ABC transporters (ABCG5 and ABCG8) cause a condition called sitosterolemia in which plant sterols accumulate in the body and may be responsible for influencing total body sterol homeostasis. In addition, other members of the subfamily G, namely ABCG1 and ABCG4, have also been shown to be involved in cellular lipid trafficking and are thought to play important roles during foam cell formation of human macrophages. By contrast, ABCG2 is a multidrug resistance transporter. In this review, we focus on the current knowledge and physiological background of the mem- bers of the subfamily G. We also present new insights on the evolutionary relationship of human and nonhuman ABCG proteins. ABBREVIATIONS USED IN THIS MANUSCRIPT ABC ATP-binding cassette (transporter/protein) ABCP placenta-specific ATP-binding cassette transporter AML acute myeloid leukemia apoE apolipoprotein E BCRP breast cancer resistance protein CETP cholesterol ester transfer protein CYP7A1 cholesterol 7α-hydroxylase EST expressed sequence tag HDL high-density lipoprotein LAL lysosomal acid lipase (or acid cholesterol ester hydrolase) LDL low-density lipoprotein LXR liver X receptor Lxra mouse liver x receptor α *Pure Appl. Chem. 74, 2021–2081 (2002). A collection of invited, peer-reviewed articles by the winners of the 2002 IUPAC Prize for Young Chemists. ‡ Corresponding author: Institut für Arterioskleroseforschung, Domagkstraße 3, 48149 Münster, Germany; Tel.: +49 (251) 83 5 20 72; Fax: +49 (251) 83 5 20 62; E-mail: stefan.lorkowski@uni-muenster.de