ESR1 and endometriosis: Oestrogen receptors play a key role in regulation of endometrial function, and it is notable that many studies have recorded dysregulation of ESR1/ESR2 with overexpression of the latter in endometriosis lesions (Table 1) which has been attributed to changes in methylation status of ESR2 (Xue et al. 2007) rather than genomic SNPs.