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During normal postnatal mammary gland development and adult remodeling related to the menstrual cycle, pregnancy, and lactation, ovarian hormones and peptide growth factors contribute to the delineation of a definite epithelial cellidentity. This identity is maintained during cellreplication in a heritable but DNA-independent manner. The preservation of cellidentity is fundamental, especialy when cels must undergo changes in response to intrinsic and extrinsic signals. The maintenance proteins, which are required for cellidentity preservation, act epigenetically by regulating gene expression through DNA methylation, histone modification, and chromatin remodeling. Among the maintenance proteins, the Trithorax (TrxG) and Polycomb (PcG) group proteins are the best characterized. In this review, we summarize the structures and activities of the TrxG and PcG complexes and describe their pivotal roles in nuclear estrogen receptor activity. In addition, we provide evidence that perturbations in these epigenetic regulators are involved in disrupting epithelial cellidentity, mammary gland remodeling, and breast cancer initiation.