PGP and cancer: One of these efflux pumps is P-glycoprotein (P-gp) that was reported in several in vitro and in vivo studies to be expressed in cancer cells, especially those of the breast (Pokharel et al., 2016; Tulsyan, Mittal & Mittal, 2016; Badowska-Kozakiewicz, Sobol & Patera, 2017; Babaer et al., 2018).