ESR1 and breast cancer: For example, the genes identified for ER-negative breast cancer included MDM4, encoding a negative regulator of TP53, which is necessary for normal breast development33; CCNE1, an important oncogene in breast cancer34,35; CASP8, encoding a regulator of apoptosis36; ATM, a known breast cancer susceptibility gene37,38; and the ER, ESR1, which encodes a critical transcription factor in breast tissue39.