Tucatinib, a selective HER-2 inhibitor, approved in 2020 for the treatment of HER-2+ breast cancer, showed high ABCG2 inhibition activity in primary leukemia blast cells and in the HL60/ABCG2 cell line, increasing cell death, Hoechst 3342 cell accumulation, and significantly reducing the leukemia stem cell population [119]. This evidence concerns the gene ERBB2 and breast cancer.