Increased expression of the immune checkpoint receptors PD‐1 and cytotoxic T‐lymphocyte protein 4 (CTLA‐4) on CD4+ and CD8+ T cells in MPN has been shown,88 and blockade of PD‐1 and CTLA‐4 can recover T‐cell reactivity against mutCALR ex vivo, and result in the production of IFNγ and TNFα.87 This evidence concerns the gene CTLA4 and myeloproliferative disorder.