Current challenges to anti-angiogenic and anti-cancer therapy include the targeting of pathways that control refractoriness and multidrug resistance (e.g. by inhibitor-mediated upregulation of ABC transporters), identification of biomarkers to predict response to therapy and elucidation of inhibitors which not only retard tumor growth but also cause tumor regression [48]–[51]. This evidence concerns the gene ABCG2 and cancer.