In MDA-MB-231 breast cancer cells, Liu et al. demonstrated that PD-L1 upregulates the expression of multidrug resistance 1/P-glycoprotein (MDR1/P-gp) through the activation of PI3K/AKT and MAPK/ERK pathways [148], thereby inducing chemotherapy resistance. This evidence concerns the gene ABCB1 and breast carcinoma.