Indeed, although 4-hydroxytamoxifen increases overall ERα protein levels, pure AEs accelerate ERα turnover through the ubiquitin–proteasome pathway in ERα-positive breast cancer cells and in extracts of rodent uterine tissues, hence, their designation as SERDs (Gibson et al. 1991, Dauvois et al. 1992, El Khissiin & Leclercq 1999, Wijayaratne & McDonnell 2001). Here, ESR1 is linked to breast cancer.