ABCB1 and breast cancer: For instance, ATP-binding cassette subfamily C member 1 (ABCC1, also known as multidrug resistance protein 1, MRP1) and ABCB1 (also called multidrug resistance protein 1 (MDR1) and P-glycoprotein (P-gp)) are highly expressed in CSCs of several types of cancer, such as glioblastoma and breast cancer, and both ABCC1 and ABCB1 mediate the efflux of a number of therapeutic compounds [15].