The promoter methylation status of ABCB1, ABCC1 and ABCG2 (Figure 1) was determined in 19 cancer cell lines: the small cell lung carcinoma DMS114 and GLC- 4; the non-small cell lung carcinoma A549, HCC827, NCI-H520, NCI-H1703 and SW1573; the colorectal carcinoma HCT116 and SW480; the breast cancer cell models MCF-7, MDA-MB-231 and ZR- 75– 1; the cervical cancer cell line KB-3-1; the ovarian carcinoma A2780; the prostate cancer cell line PC-3; the osteosarcoma MG-63 and U2-OS; the multiple myeloma U266 and the promyelocytic leukemia HL- 60. This evidence concerns the gene ABCG2 and breast cancer.