The glycolytic enzyme enolase (2-phosph-D-glycerate hydrolyase) exists as several dimeric isoenzymes (aa, ab, ag and gg) composed of three distinct subunits, a, b, and g. Three isoenzymes are found in rat brain: aa, ag and gg. The ag and gg-enolase isoenzymes are also known as neuron­-specific enolase (NSE) as these isoenzymes initially were detected in neurons and neuronendocrine cells. The NSE levels are low in health and benign subjects. Elevated levels are commonly found in patients with malignant tumors with neuronendocrine differentiation, especially small cell lung cancer and neuroblasloma. NSE are reported to be useful diagnostic marker for lung cancer, neuroblastoma, melanoma, seminoma and in injury of central nervous system.