PGP and cancer: The MDR phenotype occurs most often due to the overexpression of drug-efflux pumps in the plasma membrane of cancer cells [16], including the multidrug resistance-1 (MDR1) gene, which is known as ABCB1. It encodes for P-glycoprotein (P-gp), which is an efflux transporter that limits drugs from penetrating cells and depositing them into the extracellular space [17].