Parathyroid adenomas usually contain a thin fibrous capsule. They are highly cellular and vascular lesions. A small component of compressed non-neoplastic parathyroid tissue is seen in about 60% of cases (shown here floating above the adenoma). The genetic syndromes associated with familial primary hyperparathyroidism were mentioned in the previous slide. Molecular defects have been identified in sporadic adenomas as well: 1) Cyclin D1 gene inversion on chromosome 11 and 2) Mutations in both copies of the MEN 1 gene on chromosome 11q13.