Oestrogen also acts via specific receptors, oestrogen receptor-α (ESR1) and oestrogen receptor-β (ESR2), the latter of which has been demonstrated to be overexpressed in women with endometriosis and may play a role in disease development (Bulun et al., 2012; Simmen and Kelley, 2016; Stocks et al., 2017). Here, ESR1 is linked to endometriosis.