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Immunotherapy has been found to be efficient in a variety of cancers and, therefore, may be a promising strategy for breast cancer (BC), particularly due to the limited therapeutic options currently available for triple-negative BC (TNBC). However, heterogeneity of tumor mutation burden (TMB), immune gene expression and mismatch repair (MMR) gene activity across BC subtypes has not been well characterized. In the present study, a comprehensive analysis of TMB, expression levels of immune cell type marker genes, and expression levels of MMR-associated genes was performed. A total of 5 MMR-associated genes, including MLH1, MLH3, MSH2, MSH6 and PMS2, were analyzed. Patients that harbored at least two MMR genes with expression levels in the lower 10% percentile across the cohort were considered as potentially aberrant (lost expression). Hormone receptor (HR)-negative BC is associated with a higher TMB and immune gene expression compared with HR-positive BC [TMB, estrogen receptor (ER)-negative vs. ER-negative, 55 vs. 32, respectively; P=4.1×10−13; progesterone receptor (PR)-negative vs. PR-positive, 53 vs. 31, respectively; P<2.2×10−16]. By contrast, human epidermal growth factor receptor 2 (HER2)-negative BC tended to have a lower TMB and decreased immune gene expression compared with HER2-positive BC (TMB, HER2-negative vs. HER2-positive, 36 vs. 48, respectively; P=0.02). Furthermore, aberrant expression of MMR genes was found to be more common in HR-negative compared with HR-positive BC (P<0.001). Significantly higher expression levels of each immune marker gene of four major immune cell types were found in patients who were HR-negative compared with patients who were HR-positive (P<0.001). The findings of the present study suggest that HR-negative or HER2-positive BC exhibits elevated TMB and immunogenic activity, and immunotherapeutic options are recommended. [ABSTRACT FROM AUTHOR]