The analysis of how human cancers evade chemotherapy has revealed a rich variety of cell-based genetic changes resulting in drug resistance. One of the best studied of these genetic alterations is increased expression of an A TP-dependent plasma membrane transport system, known as P-glycoprotein, or the multidrug transporter. This transporter actively effluxes a large number of natural product, hydrophobic, cytotoxic drugs, including many important anticancer agents. This review focuses on the genetic and molecular genetic analysis of the human multidrug transporter, including structure-function analysis, pre-and posttransн lational regulation of expression, the role of gene amplification in increased expression, and the properties of transgenic and" knock-out" mice. One imporн tant feature of the MDR gene is its potential for the development of new selectable vectors for human gene therapy.