ABCB1 and cancer: In some cancer cell lines, promoter methylation of ABCB1 was found to be inversely correlated with gene expression at the mRNA and/or protein levels, indicating, that promoter methylation is an important mechanism in gene regulation of ABCB1. Analysis of MDR cell models revealed that overexpression of ABCB1 is linked to a decrease in ABCB1 promoter methylation, whereas upregulation of ABCC1 was frequently mediated by gene amplification.