Gao X. et al. investigated CD31 as an angiogenic marker, by counting the number of CD31 positive vessels within the highest expressed tumor area (MVD), and the number was significantly higher in pheochromocytomas with PASS < 4 than in those with PASS ≥ 4. This evidence concerns the gene PECAM1 and hereditary pheochromocytoma-paraganglioma.