Mutations in some genes have been found only in single patients or families (e.g., MEN1, EGLN1, EGLN2, MDH2, IDH1, BAP1, BRAF); therefore, their role in the formation of paragangliomas/pheochromocytomas cannot be reliably confirmed. This evidence concerns the gene EGLN1 and hereditary pheochromocytoma-paraganglioma.