CD4 and cancer: To address this gap in our understanding, the ‘Timer of Cell Kinetics and Activity’ (Tocky) system was developed, which can analyze the temporal dynamics of antigen-reactive conventional CD4 +Foxp3-, Treg (CD4 +Foxp3+) and CD8 +T cells (online supplemental figure 1).9 10 While Tocky has provided insights into aspects of T cell subset dynamics biology, from the perspective of Foxp3 expression as well as antigen engagement, it has not been applied to cancer immunotherapy.