MASH1 is a transcription factor that is essential for the differentiation of neuroblasts and the generation of olfactory and autonomic neurons. MASH1 also regulates the cell cycle progression, including canonical cell cycle regulators and oncogenic transcription factors. MASH1 is highly specific for diagnosing high-grade, poorly differentiated neuroendocrine carcinomas. Its expression is observed in most pulmonary neuroendocrine tumors (small cell lung carcinoma and large cell neuroendocrine carcinoma), whereas non-small cell lung cancers (adenocarcinomas, squamous cell carcinomas, and large cell carcinomas) are rarely positive.