These findings raise the possibility that CD4cyt could contribute to the otherwise unexplained immune deficiency that emerges in CTLA4 haploinsufficiency and in other immune dysregulation syndromes where autoimmunity occurs concurrently with antibody deficiency, for example, syndromes related to STAT3 gain-of-function [4, 5, 81]. This evidence concerns the gene CTLA4 and agammaglobulinemia.