CTLA4 and cancer: Combined anti‐PD‐1 and anti‐CTLA‐4 checkpoint blockade are currently among the most effective immunotherapy against various cancer types including advanced metastatic melanoma.[53] To compare this immune checkpoint therapy (ICT) with HPK1 deficiency, WT control, and MAP4K1 KO mice were injected intravenously with B16F10 cells and then treated with a control IgG or a combination of anti‐PD‐1 and anti‐CTLA‐4.