Increased levels of pro-inflammatory serum cytokines (e.g., interferon-gamma; interleukin (IL)-1beta, -2, -4, -5, -6, -8, -10, -12p70, and -13; tumor necrosis factor-alpha (TNF–α)) were demonstrated in patients with IPAH and HPAH compared to the control group. Here, TNF is linked to heritable pulmonary arterial hypertension.