Considering that endometriosis is clearly an estrogen-dependent disease, there are many reports of positive associations with numerous polymorphisms involving sex steroids production and metabolism: for example, a consistent work by Singh et al. [54] was conducted on eutopic and ectopic (ovarian) endometrium from patients with stage 3 or 4 endometriosis, comparing ectopic to eutopic endometrium; their data showed a 3–9-fold increase in intraindividual expression of CYP1A1, a 5–53-fold intra-individual increase in gamma-SYN expression, and an elevation in Estrogen Receptor β (but not α). This evidence concerns the gene CYP1A1 and endometriosis.