Cyfra21-1, or cytokeratin 19 fragment, is a soluble fragment of keratin CK19, which is primarily a tumor indicator for the differential diagnosis of non-small cell carcinoma. Keratins are a class of scaffolding proteins of epithelial cells, about 20 in total, which are not easy to be solubilized, while Cyfra21-1 is the product of the soluble fragment of cytokeratin 19 specifically binding to two monoclonal antibodies, KS19.1 and BM19.21. It is mainly found in organs such as the lungs and breast, and may be elevated when malignant changes occur in any of these tissues. Cyfra21-1 is mainly used in the differential diagnosis of non-small cell carcinoma and small cell carcinoma, with the highest positive detection rate in squamous cell carcinoma of the lung, and the level of Cyfra21-1 correlates with the tumor volume and stage, so it can also be used in the detection of the efficacy of lung cancer as well as in the prognosis and evaluation. If abnormal Cyfra21-1 results are found during physical examination, it is recommended to go to a regular hospital to avoid delaying the condition.