In 2009, Nyholt et al. [19] hypothesized shared genes between the two conditions; other studies point to the estrogen receptor 1 gene as a common genetic factor between headache (the receptor has been implicated in migraine susceptibility) [136] and endometriosis (the same receptor has been associated with endometriosis) [137,138]. The gene discussed is ESR1; the disease is endometriosis.