On the other hand, examining the contribution of the mechanism of export by p-glycoprotein, it was shown that the use of verapamil, a p-glycoprotein antagonist, can reverse the resistance of HRT-18 colorectal cancer cells to PDT with hematoporphyrin, which suggests a significant role of p-glycoprotein in reducing sensitivity to treatment [121]. This evidence concerns the gene ABCB1 and colorectal cancer.