Indeed, ABCG2—a 655-amino acid protein working as a homodimer on cellular plasma membranes (Robey et al., 2009)—is expressed not only in cancer cells but also in several normal tissues, such as brush border membranes of epithelium in the intestine and of proximal tubules in the kidney, bile canalicular membranes of the hepatocytes in the liver, luminal membranes of the mammary gland epithelium, and blood-facing membranes of the endothelial cells forming the BBB. Here, ABCG2 is linked to cancer.