Syn is a neuroendocrine marker, which is an immunological test. syn is positive for neuroendocrine tumors, and can also be used to help diagnose lung cancer. Syn is mainly found in the presynaptic vesicle membrane of neurons, and is one of the specific markers for neuroendocrine tumors. The possibility of endocrine cancer can be determined by immunohistochemistry of tumor cells to see whether the syn indicator is positive or not. In addition, according to research, syn is expressed in 32.5% of non-small cell carcinomas of the lung and up to 90% of small cell carcinomas of the lung, so it can also be used to assist in the diagnosis of lung cancer. If the immunohistochemistry test shows that syn is positive, it is recommended to have further targeted tests under the guidance of the doctor.