DNMT1 and breast cancer: To test this hypothesis, we treated breast cancer cells with 5-aza-2'-deoxycytidine (DAC), a DNMT inhibitor, and found that GATA3 mRNA levels in all BRCA1-deficient cells, HCC1937, MDA-MB-231, and BT20, were enhanced more than 2.5 fold, whereas GATA3 mRNA levels in BRCA1-proficient cells, MCF7 and T47D, were enhanced less than 1.5 fold (Figure 3A).