Achaete-scute complex homolog-1 (ASCL1), known as mASH1 in rodents and hASH1 in humans, is a basic helix-loop-helix transcription factor that is critical for neuroendocrine cell differentiation (1,2). Neuroendocrine carcinomas can arise in different sites such as lung, the gastrointestinal tract, prostate and skin (2). High grade, poorly differentiated neuroendocrine carcinomas are classified as neuroendocrine carcinomas (NECs) and are distinguished from low grade neuroendocrine tumors (NETs) (2,3). Classic neuroendocrine markers such as chromogranin and CD56 cannot distinguish NECs from NETs (2). Studies have shown that the mouse monoclonal antibody MASH1 [24B72D11.1] stains hASH1 in human tissues and can distinguish NECs from NETs in various sites (3-5). MASH1 has also been shown to distinguish large cell neuroendocrine carcinomas (LCNECs) and small cell lung carcinomas (SCLCs) from other phenotypes of lung cancer (3-5). MASH1 has also been used to differentiate SCLC from Merkel cell carcinoma (6). While not a tissue-specific marker, MASH1 may assist in distinguishing neuroendocrine carcinomas from neuroendocrine tumors in poorly differentiated cases (2).