Background: Ubiquitous mitochondrial creatine kinase (uMtCK), a mitochondrial isoenzyme of creatine kinase (CK), is a central controller of cellular energy homeostasis. Overexpression of uMtCK has been reported to be associated with a poor prognosis for several tumors. The aim of this study was to assess its association with breast cancer (BCa) and to further investigate its underlying mechanisms. Method: We first detected uMtCK expression by immunohistochemistry in human BCa tissues and assessed the association with the prognosis of patients. We then evaluated uMtCK expression in crowded and normal condition cultures of several human BCa cell lines. After two stable clones of the MDA-MB-231 cell line with high expression of uMtCK were established, cell growth, apoptosis and mitochondrial apoptotic pathway protein expression were measured in these clones. Finally, tumorigenicity of the above cells was assessed using nude mice to explore the relationship between uMtCK expression and tumor progression. Results: uMtCK expression was detected in 85.5% (47 of 55) of the invasive ductal carcinomas of breast tissue, not otherwise specified (IDC-NOS). Expression in BCa tissue was significantly associated with reduced progression-free survival (PFS; P = 0.019) and overall survival (OS; P = 0.022) of the patients. Up-regulation of uMtCK expression was identified in crowded BCa cells in culture, and the number of apoptotic cells was significantly decreased in uMtCK transfected MDA-MB-231 cell clones (P < 0.01). Stabilization of the mitochondrial membrane potential (Delta Psi m) and down regulation of cytochrome c (cyt c) and activated caspase 9, two components of mitochondrial apoptotic pathway proteins, were also identified in the same clones when cells were crowded in culture. In vivo studies revealed that the transfected tumor cells with uMtCK overexpression induced faster tumor growth in nude mice, along with accelerated animal body weight loss and a significantly lower tumor apoptotic index (AI) (P < 0.001). Conclusion: The results indicated that uMtCK expression is associated with a poor prognosis in BCa and might serve as a tumor marker. In vivo and In vitro evidence suggests that uMtCK overexpression promotes tumor growth by inhibiting apoptosis of tumor cells through stabilizing Delta Psi m and down regulating mitochondrial apoptotic pathway proteins. Exploration of therapeutic agents targeting the expression of uMtCK may have practical value for BCa patients. (C) 2012 Elsevier Inc. All rights reserved.