The ATP-binding cassette (ABC) transporters constitute a large family of 
membrane proteins, which transport a variety of compounds through the membrane 
against a concentration gradient at the cost of ATP hydrolysis. Substrates of 
the ABC transporters include lipids, bile acids, xenobiotics, and peptides for 
antigen presentation. As they transport exogenous and endogenous compounds, they 
reduce the body load of potentially harmful substances. One by-product of such 
protective function is that they also eliminate various useful drugs from the 
body, causing drug resistance. This review is a brief summary of the structure, 
function, and expression of the important drug resistance-conferring members 
belonging to three subfamilies of the human ABC family; these are ABCB1 
(MDR1/P-glycoprotein of subfamily ABCB), subfamily ABCC (MRPs), and ABCG2 (BCRP 
of subfamily ABCG), which are expressed in various organs. In the text, the 
transporter symbol that carries the subfamily name (such as ABCB1, ABCC1, etc.) 
is used interchangeably with the corresponding original names, such as 
MDR1P-glycoprotein, MRP1, etc., respectively. Both nomenclatures are maintained 
in the text because both are still used in the transporter literature. This 
helps readers relate various names that they encounter in the literature. It now 
appears that P-glycoprotein, MRP1, MRP2, and BCRP can explain the phenomenon of 
multidrug resistance in all cell lines analyzed thus far. Also discussed are the 
gene structure, regulation of expression, and various polymorphisms in these 
genes. Because genetic polymorphism is thought to underlie interindividual 
differences, including their response to drugs and other xenobiotics, the 
importance of polymorphism in these genes is also discussed.