ESR1 and breast cancer: In studies on breast cancer, but also OC cells, ERα itself was recognized as a component of the epigenetic acting MegaTrans complex, which contains histone acetyltransferases (HATs), and deposits histone acetylation marks, such as H3K27ac, at promoter or enhancer regions, and as such regulates chromatin compaction and activates transcription, particularly of estrogen-dependent genes.