Primary hyperparathyroidism (PHPT), caused by hyperplasia and/or adenomas of the parathyroids, is the most common (up to 100% after the age of 55 years) and, in the great majority of cases, the first clinical manifestation in Multiple Endocrine Neoplasia Type 1 (MEN1), a rare endocrine inherited multiple tumor syndrome caused by germline heterozygote inactivating mutations of the MEN1 tumor suppressor gene. The gene discussed is MEN1; the disease is multiple endocrine neoplasia type 1.