As indicated by the former concept, disease features frequently vary among carriers of an identical RET mutation, as well as among family members, diagnosed at different ages, with different aggressiveness, with or without associated pheochromocytoma (PHEO) and primary hyperparathyroidism (PHPT), suggesting the existence of other factors modifying the disease’s course [85]. The gene discussed is RET; the disease is hereditary pheochromocytoma-paraganglioma.