ABCG2 and cancer: Overexpression of ABC transporters in cancer is mediated by (i) increased activity of proteins involved in the MAPK (HRas, ERK1/2, JNK), PI3K/AKT, mTOR, JNK, PKC signaling pathways, (ii) activation of EGF/FGF growth factors [54,66,67,68], (iii) nuclear localization of Y-box binding protein 1 (YB-1) in solid tumors [69,70], (iv) increased COX-2 activity [71], (v) activation of VEGF2 (vascular endothelial growth factor receptor 2) by VEGF in tumor microenvironment [70], (vi) activation of nuclear receptors PXR and CAR [72,73,74] and (vii) hypoxia [75].