While de novo oestrogen synthesis via aromatase has emerged as a key steroidogenic process in the normal endometrium (Das et al. 2009, 2012, Gibson et al. 2013), activation of sulphated steroids via STS is reported to be a prominent mechanism of oestrogen activation in pathologies of the endometrium including endometrial cancer and endometriosis (Colette et al. 2011, Sinreih et al. 2017). Here, CYP19A1 is linked to endometriosis.