HMOX1 and polycythemia: Rats exposed to hypobaric hypoxia exhibit a threefold increase in HO activity in the lungs after 24 h of hypoxia, followed by suppression of HO activity back to baseline values from days 3–21 of hypoxia that coincides with the development of polycythemia, right ventricular hypertrophy and pulmonary remodeling (Carraway et al., 2000).