ABCG2 and breast cancer: Since the ABC transporters such as MDR1 and multidrug resistance-associated protein 1 (MRP1) appear upregulated in T-DM1-resistant breast cancer cells9–11, it is possible that these ABC transporters are involved in both acquired resistance to T-DM1 and cross-resistance to SOC chemotherapeutics and regulate invasive behavior of T-DM1-resistant breast cancer cells.