Abstract: Objective: To explore the application value of the 99mTc-CXC chemokine receptor 4(CXCR4) inhibitor derived from virus macrophage inflammatory protein(NT21MP) as SPECT imaging agent in mouse breast cancer.Methods: NT21MP was directly radiolabeled using pretinning method.The radiochemical purity and stability of 99mTc-NT21MP in plasma was measured by chromatographic system.The mouse model of breast cancer was established.SPECT imaging of model mice were detected by intravenous injection 99mTc-NT21MP.T / NT value was calculated,and the expressions of CXCR4 and SDF-1 in tumor and lung tissue were detected by immunohistochemistry method.Results: The radiochemical purity of 99mTc-NT21MP was 95%.It was stable in blood plasma within 2 hours,its radiochemical purity was more than 90%.99mTc-NT21MP could target to tumor tissues after injecting.T / NT reached 4.6 in 2 hours,the expression of CXCR4 and SDF-1 were high in tumour and lung tissue,respectively.Conclusions:99mTc-NT21MP is easily prepared,and high targeting,which has high application value in early diagnosis of CXCR4 positive tumor.