CTLA4 and cancer: In cancers, CTLA-4 extrinsically controls effector T-cell responses, a mechanism mediated by regulatory CD4 T-cells (Tregs), and in the context of cancer strategies to evade anti-tumour immunity, there is evidence that tumours can actively induce CTLA-4+ intratumoural Tregs to propagate an immunosuppressive milieu, thereby preventing effector T-cells from initiating successful anti-tumour responses.