CD69 and breast carcinoma: While PD-1 expression is mainly attributed to dysfunctional (exhausted) T cells in lung cancer [55], PD-1 expression in breast cancer was detected in higher frequencies in both CD103+CD69+ and CD103−CD69+ T cells compared to CD103−CD69− T cells [27], suggesting that PD-1 upregulation may be one if the first steps in TRM differentiation and effector T cell (pre)dysfunctionality in breast cancer.