Furthermore, their expression is heterogenous even between tumors of the same class and grade and can be further enhanced after drug administration, as it has been observed for doxorubicin [107,340,341], which is a common substrate of P-gp, BCRP, MRP1, MRP2, MRP3 and MRP6; this complicates the development precise strategies against the tumor. The gene discussed is ABCG2; the disease is neoplasm.