one current concept suggests that unchecked proliferation of clonally selected precursors of endothelial cells (ecs) contribute to severe pulmonary arterial hypertension (pAH). We hypothesized that clonally selected ECs expressing the progenitor marker CD117 promote severe occlusive pulmonary hypertension (PH). The remodelled pulmonary arteries of PAH patients harboured CD117 + ecs. Rat lung CD117 + ecs underwent four generations of clonal expansion to enrich hyperproliferative ecs. The resulting clonally enriched ECs behaved like ECs, as measured by in vitro and in vivo angiogenesis assays. The same primitive ECs showed a limited ability for mesenchymal lineage differentiation. Endothelial differentiation and function were enhanced by blocking TGF-β signalling, promoting bone morphogenic protein (BMP) signalling. The transplantation of the EC clones caused arterio-occlusive PH in rats exposed to chronic hypoxia. these ec clones engrafted in the pulmonary arteries. Yet cessation of chronic hypoxia promoted lung cell apoptosis and resolution of vascular lesions. In conclusion, this is to the best of our knowledge, the first report that clonally enriched primitive ECs promote occlusive pulmonary arteriopathy and severe pH. these primitive ec clones further give rise to cells of endothelial and mesenchymal lineage as directed by BMP and TGF-β signaling. Pulmonary arterial hypertension (PAH) remains a devastating disease with dismal prognosis. The pulmonary arteries of PAH patients have lesions of varying severity including formation of neointima, which obstructs the normal blood flow, and complex multicellular plexiform lesions. A current concept suggests that dysregulated programmed cell death, or apoptosis of ECs causes unchecked proliferation of a subset of apoptosis-resistant ECs 1,2. The ECs retain a hyperproliferative and apoptosis-resistant phenotype even in culture 3. These abnormal ECs in the plexiform lesions of PAH patients may originate from primitive endothelial-like stem cells by clonal selection 4. This concept of primitive cells as source of aberrant ECs in PAH is supported by the findings of markers of primitive stem-like cells in the lung vascular lesions 5-7. Likewise, hyperproliferative pulmonary artery smooth muscle cells may also be derived from progenitor cells in PAH 8,9. During vascular development, primitive ECs