['CYFRA 21-1 measures soluble cytokeratin-19 fragments in serum and is a useful marker for lung carcinoma, especially squamous cell carcinoma (SCC). The authors conducted this study to determine the significance of CYFRA 21-1 in patients with esophageal SCC.', "Expression and production of cytokeratin-19 in the authors' six established esophageal SCC cell lines were determined by immunocytochemical staining and enzyme-linked immunoadsorbent assay, respectively. The correlation between serum CYFRA 21-1 levels and expression of cytokeratin-19 in human tumors was investigated by immunohistochemical staining. The correlation between serum CYFRA 21-1 levels and clinicopathologic factors was examined in 48 patients with esophageal SCC, as were SCC antigen and carcinoembryonic antigen (CEA).", 'Of the 6 cell lines, 5 lines expressed cytokeratin-19 in their cytoplasm and produced soluble cytokeratin-19 fragments. Twenty-three of 48 patients had elevated CYFRA 21-1 levels (>3.5 ng/mL), whereas none of the reference group (consisting of healthy volunteers or patients with benign disease) showed positive levels. The specificity, sensitivity, and accuracy of CYFRA 21-1 were 100%, 47.9%, and 66.7%, respectively. CYFRA 21-1 showed significantly higher sensitivity and accuracy than SCC antigen or CEA (P < 0.05). Univariate analysis revealed that CYFRA 21-1 levels correlated with disease progression (including tumor size, tumor depth, and pTNM stage), resectability, and curability. There was a significant association between the level of CYFRA 21-1 and its intensity of immunohistochemical staining in vitro as well as in vivo.'] Is cYFRA 21-1 a useful marker for esophageal squamous cell carcinoma?