Patients with CTLA‐4 haploinsufficiency develop autoimmune thrombocytopenias and abnormal lymphocytic infiltration of non‐lymphoid organs, including the lungs, brain, and GI tract, resulting in enteropathy.128 CTLA‐4 haploinsufficiency has been observed to have incomplete penetrance. Here, CTLA4 is linked to autoimmune thrombocytopenia.