Consequently, the possibility that the enhanced pain perception masked the effect of ch-OSA in women in the present study cannot be ruled out given the predominance of postmenopausal women (98%) with low endogenous estrogen levels, of which only 7% was on hormone replacement therapy (HRT), a contraceptive, a selective estrogen receptor modulator (SERM) or phytoestrogens. This evidence concerns the gene ESR1 and obstructive sleep apnea syndrome.