HPS is a result of excessive vasodilatation into the pulmonary vasculature caused by numerous molecules—nitrogen (NO), carbon monoxide (CO), and endothelin-1 (ET1)—and monocyte overstimulation, which predisposes individuals to vascular endothelial growth factor (VEGF) activation and neo-angiogenesis. This evidence concerns the gene EDN1 and Hermansky-Pudlak syndrome.