By DNA methylation analysis of the ABCB1, ABCC1, and ABCG2 promoters in three breast cancer cell lines (MCF7, luminal A subtype; ZR-75-1, luminal B subtype; MDA-MB-231, triple negative subtype), the ABCC1 promoter has been found to be unmethylated in each of the cell lines [142]. Here, ABCG2 is linked to breast cancer.