We have shown that ECRG4 suppressed Fas-induced apoptosis in Jurkat cells. ECRG4 mRNA expression was ubiquitously detected in normal adult human tissues, suggesting that ECRG4 plays a major role in human tissues. ECRG4 mRNA expression was down-regulated in tumor cells. Expression of ECRG4 suppressed cell growth. We established an anti-ECRG4 monoclonal antibody. Our immunohistochemical analysis demonstrated that ECRG4-positive cells tended to be distributed in the region that was negative for Ki-67 in esophageal squamous cell carcinoma tissues. There was a significant inverse correlation between ECRG4 expression and Ki-67 labeling index in esophageal squamous cell carcinoma. This study provides the first functional evidence for an association of endogenous expression of ECRG4 with cell proliferation. ECRG4 is a candidate tumor suppressor gene that might be involved in the proliferation of esophageal squamous cell carcinoma.