P-glycoprotein 1 (multidrug resistance protein 1, MDR1) is an energy-dependent efflux pump responsible for decreased drug accumulation in multidrug-resistant cells. MDR1 is a member of the superfamily of ATP-binding cassette (ABC) transporters. Genetic variation in MDR1 are the cause for patients who do not respond to drug treatment. In mouse, the MDR gene family includes three or more related but distinct cellular genes. MDR1A is also called P-glycoprotein 3.