Overexpression of three proteins belonging to the ABC transporter superfamily has been identified in CSCs and extensively studied in PC, including P-glycoprotein (P-gp, also known as ABCB1 or multidrug resistance (MDR) protein 1 (MDR1)), breast cancer resistance protein (BRCP or ABCG2), and the MDR-associated protein 1 (MRP1 or ABCC1) [80]. This evidence concerns the gene ABCG2 and pachyonychia congenita.