A subfamily of transmembrane proteins from the superfamily of atp-binding cassette transporters that are closely related in sequence to P-GLYCOPROTEIN. When overexpressed, they function as ATP-dependent efflux pumps able to extrude lipophilic drugs, especially antineoplastic agents, from cells causing multidrug resistance (drug resistance, multiple). Although p-glycoproteins share functional similarities to multidrug resistance-associated proteins they are two distinct subclasses of atp-binding cassette transporters, and have little sequence homology.