FLT1 and pulmonary arterial hypertension: Subsequently, based on differential gene expression in both HSC and endothelial clusters, previous utility as a biomarker in PAH, and participation in the upregulated VEGF and HIF-1 signaling pathways noted above, we selected a member of the VEGF signaling cascade (Vascular Endothelial Growth Factor Receptor 1, FLT1) as a promising candidate biomarker for further validation26,27 (Supplementary Table 1).