Small integrin-binding ligand N-linked glycoproteins (SIBLINGs) constitute a family of extracellular matrix proteins involved in bone homeostasis. Their pattern of expression has been primarily reported in bone and tooth and, more recently, in several cancer types. Dentin matrix protein 1 (DMP1), a SIBLING family member, expression was investigated by immunohistochemistry in a retrospective series of 148 primary human breast cancers. Correlations between DMP1 expression levels in the tumors and clinicopathologic features, bone metastases development and relapse of the disease were examined. DMP1 was expressed by 63.5% of the breast tumors analyzed. Significant inverse associations were found between DMP1 expression levels and the size and grade of the tumors (both, P < 0.0001). High DMP1 expression levels in the primary breast lesions were associated with a lower risk of subsequent development of skeletal metastases (P = 0.009). Patients with tumors expressing high levels of DMP1 had a significantly higher disease-free survival rate than those with low DMP1-expressing tumors (P = 0.0062). When DMP1 expression was examined in breast cancer cell lines, we found that non invasive MCF-7 and T47-D cells expressed higher levels than highly invasive MDA-MB-231 and Hs578T cells. Moreover, the specific inhibition of DMP1 expression in MCF-7 cells using siRNAs promoted significantly their migratory capability. Our data implicate for the first time DMP1 expression in breast cancer progression and bone metastases development.