ABCG2 and cancer: Another epigenetic factor‐linked overexpression of ABC transporters in cancer is the hypomethylation of the transporter gene promoter, and evidence has shown that agonists of DNA methyl transferases can induce the hypermethylation of the ABCG2 promoter resulting in the lower ABCG2 expression and increased intracellular concentration of substrate drugs.138, 139