Resistance to chemotherapy occurs in cancer cells because of intrinsic or acquired changes in expression of specific proteins. We have studied resistance to natural product chemotherapeutic agents such as doxorubicin, Vinca alkaloids, and taxol, and to the synthetic drug cisplatin. In both cases, cells become simultaneously resistant to multiple drugs because of reductions in intracellular drug concentrations. For the natural product drugs this cross-resistance is due to expression of an energy-dependent drug efflux system (ABC transporter) known as P-glycoprotein (P-gp), the product of the MDR1 or ABC B1 gene. For cisplatin, cross-resistance to methotrexate, some nucleoside analogs, heavy metals, and toxins is due to a reduction in drug influx resulting from a pleiotropic defect in uptake systems. Recent evidence suggests a global defect in endocytosis in these cisplatin resistant cells and defects in intracellular protein trafficking and the cytoskeleton. Single-step cisplatin resistant mutants show a defect in protein trafficking which results in accumulation of cell surface receptors/transporters/channels in the cytoplasm; a putative cisplatin carrier/channel is presumed to be among these mislocalized proteins resulting in decreased cisplatin uptake. At higher levels of resistance, after multiple steps of selection in cisplatin, there is increased methylation of genes for binding proteins (e.g., folate binding protein) and cytoskeletal proteins, among others. This hypermethylation, reversible by treatment with deoxyazacytidine, results in decreased RNA transcription of genes responsible, at least in part, for the cisplatin resistance phenotype. Studies on mechanism of action of P-gp have focused on the manner in which many different substrates and inhibitors are recognized by the transporter, how substrate interaction results in activation of ATPase, and how ATPase results in drug translocation and efflux. These studies and others have led to the conclusion that there are multiple, probably overlapping sites for interaction of substrates and inhibitors primarily formed by TM segments from both the amino-terminal (TM5,6) and carboxy-terminal (TM11,12) halves of P-gp and that activation of ATPase results in a reduction of substrate binding to P-gp. Studies on the normal function of P-gp suggest that it is involved in normal uptake and distribution of many drugs. Common polymorphic variants of P-gp have been detected, but coding polymorphisms do not appear to alter the drug transport functions of P-gp. To explore the possibility that other members of the ABC family of transporters may be involved in drug resistance in cancer, we have developed real-time PCR and microarray technology for detection of most of the 48 known ABC transporters; these techniques have been used to correlate expression of novel ABC transporters in cancer cell lines of known drug resistance. Use of the MDR1 gene as a dominant selectable marker in gene therapy has focused on the development of SV40 as a vector for delivery of MDR1. Using recombinant SV40 capsid proteins, it is possible to package DNA in vitro, including P-gp and green fluorescent protein (GFP) containing vectors. Transduction of P-gp and GFP using in vitro packaged DNA is highly efficient in many different cell types including lymphoid cells, liver cells, and keratinocytes, and allows transfer of up to 15 kb of DNA without the need for SV40 sequences in the packaged DNA. This approach offers promise for transfer of P-gp into hematopoietic and other cells for gene therapy.