CA9 and obstructive sleep apnea syndrome: The expression of CAIX, as a biomarker of hypoxia in pregnancy, was evaluated by Ravishankar et al., in physiological placentas and in placentas of women with obstructive sleep apnea (OSA), showing positive membranous staining in the chorionic plate mesenchyme layer and weak cytoplasmic staining in amniotic epithelium (Ravishankar et al., 2015).