It is more potent than that of estrogen.13 16αOHE1 stimulates cell proliferation and has been implicated in experimental PAH,1 as well as having genotoxic effects in other systems.14 Molecular processes underlying these effects are unclear, although reactive oxygen species (ROS) may be important because estrogen, through its estrogen receptor (ER), and estrogen metabolites have been shown to cause cell proliferation through redox-sensitive processes.15 The gene discussed is ESR1; the disease is pulmonary arterial hypertension.