Familial isolated hyperparathyroidism can be caused by mutations in the MEN1, CDC73, or CASR gene.  The MEN1 gene provides instructions for producing a protein called menin. Menin acts as a tumor suppressor, which means it normally keeps cells from growing and dividing (proliferating) too rapidly or in an uncontrolled way. In familial isolated hyperparathyroidism, MEN1 gene mutations result in an altered menin protein that is no longer able to control cell growth and division. The resulting increase in cell proliferation leads to the formation of an adenoma involving one or more parathyroid glands. Overproduction of parathyroid hormone from these abnormal glands stimulates the release of excess calcium into the blood, leading to the signs and symptoms of familial isolated hyperparathyroidism. It is unclear why this condition affects only the parathyroid glands.  The CDC73 gene provides instructions for making the parafibromin protein, which is also thought to act as a tumor suppressor. Parafibromin is likely involved in regulating the activity of other genes (gene transcription) and in cell proliferation. CDC73 gene mutations that cause familial isolated hyperparathyroidism likely result in decreased activity of the parafibromin protein. The loss of parafibromin's tumor suppressor function can lead to the development of parathyroid adenoma or, rarely, parathyroid carcinoma.  The CASR gene provides instructions for producing a protein called the calcium-sensing receptor (CaSR), which helps regulate the amount of calcium in the body, in part by controlling the production of parathyroid hormone. Calcium molecules attach (bind) to CaSR, turning on (activating) the receptor. When calcium binds to the CaSR protein in cells of the parathyroid gland, the activated receptor sends signals that block the production and release of parathyroid hormone. Without parathyroid hormone, calcium is not released into the blood. CASR gene mutations associated with familial isolated hyperparathyroidism lead to the production of a less sensitive CaSR that requires an abnormally high concentration of calcium to trigger signaling. As a result, parathyroid hormone is produced even when the concentration of calcium in the blood is elevated, allowing the calcium levels to continue to rise. A small number of individuals with CASR-related familial isolated hyperparathyroidism have enlarged parathyroid glands, and overproduction of parathyroid hormone from these abnormal glands further contributes to the elevated calcium levels in the bloodstream. The excess calcium causes the characteristic features of this condition.  Mutations in the MEN1 gene and the CDC73 gene are involved in other conditions in which hyperparathyroidism is just one of many features. However, some people with mutations in these genes have only signs and symptoms related to hyperparathyroidism (isolated hyperparathyroidism) without the additional features of these other conditions. While some individuals later develop additional signs and symptoms of the other conditions, others do not. Familial isolated hyperparathyroidism may be a milder variant or early form of the other conditions.  In many individuals with the signs and symptoms of familial isolated hyperparathyroidism, a mutation in the MEN1, CDC73, or CASR gene has not been identified, indicating that other genes may be involved in this condition. The genetic cause of these cases is unknown.