Daintain, a novel bioactive peptide produced and secreted by macrophages, was expressed in breast tumor tissues. The spatial distri- butions of daintain in 66 breast tumor specimens were investigated with immuno-histochemistry method. Reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization inspection system were also used to detect daintain in 45 cases of malignant breast tumors. The final results show that 93% high positive responses to daintain on breast can- cer tumors. RT-PCR demonstrated that, no smear of daintain transcripted in benign tissues was found, and light smear in peri-cancer tissue was observed. Distribution of daintain was distinguishable among benign tissues, hyperplasia tissues, immature hyperplasia and invasive breast cancer, which can be used to mark the progression of the malignant lesion development. We conclude that the expression of daintain is up-regulated in breast cancers, which indicates that the peptide is closely associated with the disease progression. So daintain could be used as the biomarker for detecting breast cancer. Daintain, a novel bioactive peptide produced and secreted by macrophages, was expressed in breast tumor tissues. The spatial distri- butions of daintain in 66 breast tumor specimens were investigated with immuno-histochemistry. Reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization inspection system were also used to detect daintain in 45 cases of malignant breast tumors. The final results show that 93% high positive responses to daintain on breast can- cer tumors. RT-PCR demonstrated that, no smear of daintain transcripted in benign tissues was found, and light smear in peri-cancer tissue was observed. Distribution of daintain was distinguishable among benign tissues, hyperplasia tissues, immature hyperplasia and invasive breast cancer, which can be used to mark the progression of the malignant lesion development. We conclude that the expression of daintain is up-regulated in breast cancers, which indicates that the peptide is closely associated with the disease progression. So daintain could be used as the biomarker for detecting breast cancer.