In support of this hypothesis, in this study we have been able to determine, using autoradiography, the ET receptor densities within the smaller vessels of the tertiary structures of the lung demonstrating, in agreement with Davie et al. 2002, a ratio 50:50 for ETA to ETB receptors with a significant increase in the ETA subtype in PAH vessels compared to control lung vessels. The gene discussed is EDNRA; the disease is pulmonary arterial hypertension.