PGP and neoplasm: One of the most important mechanisms by which tumor cells resist to cytotoxic effects of a variety of chemotherapeutic drugs (including vinblastine, doxorubicine, etoposide and teniposide, as well as many other cytotoxic agents) is overexpression of the mdr1 gene and its product, P-glycoprotein (P-gp) [18].