ABCG2 and neoplasm: The formation of tumor resistance is complex and based on intrinsic and acquired resistance mechanisms which include increased efflux of chemotherapeutics (e.g., by ABC transporters), increased DNA repair, mutation or alteration of drug targets, epigenetic mechanisms such as epigenetic regulation of gene expression and/or of protein drug targets, induction of senescence, factors in the tumor microenvironment, and epithelial-to-mesenchymal transition[4,5].