Enormous evidence is also available that the methylation of ABCB1 promoter is inversely correlated with the mRNA and protein expression of the ABCB1 in prostate cancer and leukemia, while the association of the ABCG2 promoter methylation state and its mRNA and protein expression level is extensively varied by cancer types (Zappe and Cichna-Markl, 2020). Here, ABCG2 is linked to cancer.