Since P-gp-MDR1 function is also pH-dependent (Varma and Panchagnula, 2005), and the increased lysosomal pH can reduce P-gp-MDR1 activity on lysosomal membrane, it leads to the increased cytotoxic effects of MMAE on the tumor cells that highly express P-gp-MDR1. This evidence concerns the gene ABCB1 and neoplasm.