CDKN1B and pulmonary arterial hypertension: Vascular lesions in patients with severe PAH are characterized by the existence of two types of EC phenotypes: (1) normal quiescent apoptosis-sensitive ECs located in peripheral areas, which have a high expression of p27kip1 (a marker of low growth); and (2) highly proliferative apoptosis-resistant cells in the central core of the vascular lesion, which have low expression of p27kip1 and increased expression of HIF-1α, VEGF protein, and VEGF-2 receptor [158,159].