Insulinoma-associated protein 1 (INSM1) is a developmentally regulated zinc-finger transcription factor. It localizes to the nucleus and is expressed in embryonic tissues undergoing neuroendocrine differentiation. INSM1 is not expressed in normal adult tissues but can be found highly expressed in neuroendocrine tumors. INSM1 is positive in 95% of lung small cell carcinoma and 91% of lung large cell neuroendocrine carcinoma, compared with 75% and 78% with the combined panel of traditional neuroendocrine markers (synaptophysin, chromogranin, and CD56). INSM1 stains 100% of the atypical carcinoids, typical carcinoids, and paragangliomas, but only 3% of adenocarcinomas and 4% of squamous cell carcinomas. Therefore, INSM1 is sensitive and specific to be a single first-line pan-neuroendocrine marker.