ABCG2 and neoplasm: Tumor chemoresistance mechanisms remain not well characterized, but some genetic mutations located on coding-genes for repairing proteins could be responsible of chemoresistance phenomena, alterations on cellular apoptosis process through the ATR pathway, and/or p53-MAPK cell signaling pathways [63, 53], or by the functional activity of the membrane transporter proteins ABCG2/BCRP [64].