The most important findings of our study are: (1) the expression level of efflux pump reflects the resistance to specific anticancer drug thus it should be assessed before the initiation of chemotherapy; (2) in canine mammary cancer PGP and MRP1 are responsible for vinblastine resistance, PGP, BCRP, MRP1 and MRP3 are cisplatin transporters whereas BCRP is a transporter of cyclophosphamide; (3) treatment of cells using siRNA targeting efflux pumps significantly increases cancer susceptibility to anticancer drug and allows to decrease the effective dose of anticancer drug. The gene discussed is ABCG2; the disease is cancer.