Further studies revealed that the P-gp/MDR1 expression level is up regulated in cancer cell in response to the chemotherapeutic agents and the deletion of P-gp (MDR1 knockout mice) display significant enhancements in the accumulation of P-gp substrates within the cell (Goldstein et al., 1989; Noonan et al., 1990; Gottesman, 2002). Here, PGP is linked to cancer.