ABCB1(also known as P-gp/ P-glycoprotein, MDR1/multidrug-resistance 1) is a plasma membrane protein first discovered in multidrug resistant tumor cells. ABCB1 can extrude a large number of medically relevant compounds from cells and causes drug resistance. ABCB1 is expressed in a variety of human cancers as well as normal tissues including brain, and placenta. Overexpression of ABCB1 correlates with a negative prognosis in several types of cancer.