Muscle intestine stomach expression 1 (MIST1), also named BHLHA15, is a secretory cell transcription factor that belongs to the basic helix-loop-helix superfamily. MIST1 is mainly expressed in serous secreted cells and essential for the development and maturation of secretory cells. Secretory cells lacking MIST1 exhibit structural and functional defects, including alterations in cytoskeletal organization and cell polarity and improper cell-cell junction. In healthy pancreata, MIST1 facilitates cell-to-cell communication, promotes acinar cell polarity, controls Ca2+ flux, promotes unfolded protein response (UPR) homeostasis, aids in vesicle assembly, and is required for regulated exocytosis. MIST1 was downregulated in pancreatic cancer tissues compared with normal or adjacent pancreatic tissues and lower MIST1 expression in pancreatic cancer cell lines than normal acinar cells (PMID:29859299).