KRT19 (type 1 cytokeratin 19) is 44kDa intermediate filament (IF) protein that is part of the Keratin family of proteins. Cytokeratins play a critical role in differentiation and tissue specialization and function to maintain the overall structural integrity of epithelial cells and have been found to be useful markers of tissue differentiation. KRT19 is an important biomarker for breast cancer cells, tumor cells of the lymph nodes, peripheral blood, and bone marrow. Cytokeratin 19 can be expressed with KRT8 and KRT18 to differentiate cells of epithelial lines from hematopoietic tumor cells in the peripheral blood.