Syn (synaptophysin), also known as the principal synaptic vesicle protein p38, is a protein that in humans is encoded by the SYP gene. Anti-synaptophysin reacts with neuroendocrine cells of the human adrenal medulla, pituitary gland, thyroid gland, lung, pancreas and gastrointestinal mucosa. Positive staining is seen in neurons of the brain. This antibody identifies normal neuroendocrine cells and neuroendocrine neoplasia. A diffuse, finely granulated, cytoplasmic staining is observed, which probably correlates with the distribution of the antigen within neurosecretory vesicles. Anti-synaptophysin is an independent, broad marker for neuronal and neuroendocrine differentiation.