p-glycoprotein 1 (permeability glycoprotein, abbreviated as p-gp or pgp) also known as multidrug resistance protein 1 (mdr1) or atp-binding cassette sub-family b member 1 (abcb1) or cluster of differentiation 243 (cd243) is an important protein of the cell membrane that pumps many foreign substances out of cells. more formally, it is an atp-dependent efflux pump with broad substrate specificity. it exists in animals, fungi, and bacteria, and it likely evolved as a defense mechanism against harmful substances.