Objective: There are small studies on expression of mammaglobin and GCDFP-15 for detection of basal-like triple negative breast cancer. This type of cancer has shorter survival and higher mortality rate. There must be reliable markers for detecting this type of tumor especially in metastatic cases with unknown origin.

Material and Method: In this study we assessed 66 paraffin blocks of breast cancers previously diagnosed as triple negative subtype in Mehr hospital (Tehran, Iran) by tissue microarray and immunohistochemistry technique for expression of mammaglobin and GCDFP-15.

Results: GCDFP-15 was positive in 12 cases (18.2%) and the other 54 (81.8%) cases remained negative. Mammaglobin was positive only in 6 cases (9.1%) and the remaining 60 (90.9%) cases were negative.

Conclusion: According to recent studies and our findings, there is no useful immunohistochemical marker for detection of breast source in cases of metastatic triple negative breast cancer with unknown origin and we must try hard to discover more accurate immunohistochemical markers for these highly metastatic breast cancers.