Numerous reasons for MDR development have been presented: reduced drug uptake, drug inactivation, drug target mutations, evading cell death (mainly apoptosis), the self-renewing of cancer resulting from the presence of so-called tumor stem cells, the delineation of features of the tumor microenvironment, including immunosuppression, and the main process: increased drug efflux caused by the upregulation of the expression of ABC transporters that recognize and catalyze the efflux of diverse anticancer drugs from the cells [1,2]. This evidence concerns the gene ABCG2 and cancer.