LOH and p27 deficiency was observed in a bronchial carcinoid and cervical neuroendocrine carcinoma from patients with MEN4, supporting that CDKN1B may act as a tumor suppressor gene; however, LOH was not observed in a parathyroid adenoma from one of these cases, and reduced expression of p27 was observed in a LOH-negative tumor, indicating that CDKN1B haploinsufficiency may lead to tumor development (13, 15). The gene discussed is CDKN1B; the disease is parathyroid gland adenoma.