DNMT1 and breast cancer: For instance, treatment of the human breast cancer cell line, MCF-7 for 12 h and 24 h with endosulfan at 10 μM led to significant upregulation of both DNMT1 and de novo DNMT3A and DNMT3B. Moreover, the total intracellular histone deacetylase (HDAC) activity was found to be significantly increased, which was correlated with the upregulation of class I HDACs (HDAC 1 and HDAC3), while no significant alteration in the other HDAC classes was observed in MCF-7 cells [32].