AIB1 is believed to be fundamental to ERα signaling in the development and progression of breast cancer; it is amplified or overexpressed in approximately two thirds of human breast cancers and overexpression of AIB1 in the mammary epithelium of mice leads to the formation of mammary adenocarcinomas, 85% of which express ERα [20], [21]. Here, NCOA3 is linked to breast adenocarcinoma.