The type I cytokeratins consist of acidic proteins which are arranged in pairs of heterotypic keratin chains.

It is specifically found in the embryonic periderm, the transiently superficial layer that envelops the developing epidermis.

[6] CYFRA 21-1, a soluble fragment of KRT19, is a tumor marker of various types of cancer, including lung, breast, stomach, pancreas, ovary.

[7][8] Due to its high sensitivity, KRT19 is the most used marker for the RT-PCR-mediated detection of tumor cells disseminated in lymph nodes, peripheral blood, and bone marrow of breast cancer patients.

False positivity in CYFRA 21-1 / KRT19 RT-PCR studies include: Moreover, Ck-19 is widely applied as post-operative diagnostic marker of papillary thyroid carcinoma.