Elevated circulating levels of leptin were observed in idiopathic pulmonary arterial hypertension (IPAH) and connective tissue disease-associated pulmonary arterial hypertension (CTD-PAH) patients and murine models of PH [14,34,35], and the expressions of both Ob and ObR were also found to be upregulated in the pulmonary vasculature of IPAH patients and experimental PH mice [14,36]. This evidence concerns the gene LEP and pulmonary arterial hypertension.