ABCB6 and cancer: Chemoresistance of cancer stem cells may be provided by several factors, namely, by the activation of drug efflux mechanisms (ABC family transporters) and the multidrug resistance P-glycoprotein (P-gp), and by the overexpression of DNA repair mechanisms, including homologous recombination, non-homologous end joining and base excision repair through increased poly (ADP-ribose) polymerase 1 (PARP1) activity [42].