A number of experimental and clinical studies implicated the aggravating roles of estrogen in the pathogenesis of PAH, relating to tryptophan hydroxylase-1, 5-hydroxytryptamine, serotonin transporter, cytochrome P450 1B1, and mutations in bone morphogenetic protein receptor type 2 [20–23]. The gene discussed is CYP1B1; the disease is pulmonary arterial hypertension.