To date, the most widely studied cellular mechanisms of tumor resistance are those associated with drug efflux mechanisms involving members of the adenosine triphosphate (ATP)-binding cassette (ABC) membrane transporter family, most importantly P-glycoprotein (Pgp), but also multidrug-resistant protein 1 (MRP1) and homologs (MRP2-6) and breast cancer resistance protein (BCRP). The gene discussed is PGP; the disease is neoplasm.