CTLA4 and cancer: Immune checkpoint blockers (ICBs) have transformed cancer treatment by inhibiting the interactions between receptor‐ligand pairs that limit the immune response—so‐called immune checkpoints, including CTLA‐4 and PD‐1/PD‐L1.[1, 2] ICB therapy has produced durable responses in patients with advanced melanoma and non‐small cell lung cancer, among other cancers.[3, 4] Although extremely encouraging, not all patients benefit, and a better understanding of immune function and checkpoint control is needed to improve outcomes.[5]