It is the overexpression of ABCG2 in a variety of cell lines and tissues that becomes an essential MDR factor in solid tumors, such as breast cancer, colon cancer, gastric carcinomas, hepatocellular carcinoma, endometrial carcinoma, small cell lung cancer, melanoma, ovarian, gastric, and intestinal cancers [44]. This evidence concerns the gene ABCG2 and breast cancer.