Abstract: Objective To investigate the relationship between the expression level of chemokine ligand 9 (CXCL9) mRNA and clinicopathological features and prognosis of triple-negative breast cancer (TNBC).Additionally,this research aims to analyze the relationships of CXCL9 with infiltration degree of tumor-infiltrating lymphocytes and the expression of common immune checkpoints．Methobs The expression levels of CXCL9 mRNA in tissues obtained from TNBC and neighboring normal tissues was detected by real-time fluorescence quantitative PCR,and the correlation between the levels of CXCL9 mRNA expression and the clinical outcomes of patients with TNBC was investigated.The correlation between CXCL9 mRNA and the mRNA of Immune cells and common Immune checkpoints was analyzed by using Tumor Immune Estimation Resource (TIMER) database.Results The expression level of CXCL9 mRNA was higher in TNBC tissues and lower in paracancerous tissues（P<0.05）.The proportion of patients with tumor size ≤ 2 cm,no lymph node metastasis and TNM stage Ⅰ-Ⅱ  in patients with high expression of CXCL9 was higher than those with low CXCL9 expression (P<0.05).The prognosis of TNBC patients with high expression of CXCL9 mRNA is better than that of patients with low expression（P<0.05）.CXCL9 mRNA related genes were enriched in immune response related pathways.CXCL9 mRNA expression was positively correlated with B cells,neutrophils，dendritic cells,CD8+T cells and CD4+T cells,and negatively correlated with tumor purity.The mRNA levels of CXCL9 exhibited a positive correlation with the mRNA levels of CTLA4,PD-1，PD-L1,and LAG3.Conclusion High expression of CXCL9 mRNA in TNBC is a predictor of a good prognosis for patients. The immune microenvironment of TNBC with high expression of CXCL9 mRNA is more abundant．

Key words: chemokine ligand 9 mRNA, triple negative breast cancer, bioinformatics analysis, tumor microenvironment, immune cells, prognosis