Next, to define whether the dysfunction of CD4+ and CD8+ T cells by RNase1 requires its ribonucleolytic activity, we expressed RNase1 (R1) and a catalytically inactive RNase1 mutant (R1‐H12A)[6b] in the low‐endogenous RNase1 expressing MDA‐MB‐231 breast cancer cells (Figure S2F, Supporting Information). Here, CD8A is linked to breast carcinoma.