The Synaptophysin (SYN) gene is located on the short arm of the X chromosome. Mutations in this gene are associated with X-linked mental retardation (XLMR). The SYN gene product is a 38 kD glycoprotein that is localized to the membrane of synaptic vesicles. Using immunohistochemistry, synaptophysin can be identified in a range of neural and neuroendocrine tissues, including neurons of the central nervous system, cells of the adrenal medulla, thyroid and pancreatic islets. As a specific marker for these tissues, it can be used to identify tumors originating from them, such as neuroblastoma, retinoblastoma, phaeochromocytoma, carcinoid, small-cell carcinoma, medulloblastoma and medullary thyroid carcinoma.