KRT19 is a type 1 keratin, an intermediate filament that provides structural support and has cell cycle regulatory functions in epithelial cells. KRT19 is a common epithelial marker with cytoplasmic positivity in immunohistochemistry. It is used to identify neoplastic epithelial cells in a number of cancers, including breast carcinomas, thyroid papillary carcinomas, cholangiocarcinomas, endometrial cancers, urothelial and collecting duct renal cancers, Paget’s disease, pancreatic cancers, and epithelial odontogenic tumors. Additionally, it is a putative cancer stem cell marker in hepatocellular carcinoma. In normal tissue, KRT19 is positive in epithelia of the breast, GI tract, kidney, bile ducts, pancreatic ducts, salivary and sweat glands, prostate epithelium, placental trophoblasts, uterus, and tonsillar squamous epithelium. References: Clin Cancer Res. 2015 Jul 1;21(13):3081-91, PMID: 25820415; Int J Mol Sci. 2018 May; 19(5): 1423, PMID: 29747452; Cancers (Basel). 2019 Jan; 11(1): 99, PMID: 30650643