NSE (Neuron Specific Enolase) is an enzyme marker found in neurons and neuroendocrine cells, commonly used to assess neuroendocrine tumors and certain neural conditions. The test measures NSE levels in plasma, aiding in the diagnosis, monitoring, and prognosis of small-cell lung cancer, neuroblastoma, and other neuroendocrine tumors. Elevated NSE levels may indicate tumor activity or neural damage.