In immunotherapy naïve patients, RNAseq profiling has demonstrated an enrichment in cytolytic T cell-associated programs in MMRd endometrial tumors compared to MMRp tumors, such as an increase in tumor-reactive T cell populations highly expressing PD-1 and CD39 molecules (79) and T cells with high perforin (PRF1) expression (80). The gene discussed is PRF1; the disease is endometrium neoplasm.