A small proportion of patients with FIHP-causing parathyroid tumors bear germinal inactivating mutations of the Multiple Endocrine Neoplasia type 1 (MEN1) gene or of the Cell Division Cycle 73 (CDC73) gene, presumably being incomplete clinical expressions of Multiple Endocrine Neoplasia type 1 (MEN1) syndrome or Hyperparathyroidism-Jaw Tumor (HPT-JT) syndrome, respectively. The gene discussed is CDC73; the disease is familial isolated hyperparathyroidism.