Purpose: Estrogen-responsive finger protein (Efp) is amember ofRINGfinger-B box-Coiled Coilfamily and is also a downstream target of estrogen receptor a. Previously, Efp was shown tomediate estrogen-induced cell growth, which suggests possible involvement in the developmentof human breast carcinomas. In this study, we examined expression of Efp in breast carcinomatissues and correlated these findings with various clinicopathologic variables.Experimental Design: Thirty frozen specimens of breast carcinomas were used for immunohistochemistryand laser capture microdissection/real-time PCR of Efp. Immunohistochemistryfor Efp was also done in 151breast carcinoma specimens fixed with formalin and embedded inparaffinwax.Results: Efp immunoreactivity was detected in breast carcinoma cells and was significantlyassociated with the mRNA level (n = 30). Efp immunoreactivity was positively associated withlymph node status or estrogen receptor a status and negatively correlated with histologic gradeor 14-3-3j immunoreactivity (n = 151). Moreover, Efp immunoreactivity was significantly correlatedwith poor prognosis of breast cancer patients, and multivariate analyses of disease-freesurvival and overall survival for151breast cancer patients showed that Efp immunoreactivity wasthe independentmarker.Conclusions: Our data suggest that Efp immunoreactivity is a significant prognostic factor inbreast cancer patients. These findings may account for an oncogenic role of Efp in the tumorprogression of breast carcinoma.