CTLA4 and hyperinsulinemic hypoglycemia, familial, 4: Patients with a CTLA‐4 deficiency or IPEX have chronic lymphadenopathy and their B cell phenotype is similar, albeit often more severe, to those observed in many patients with Helios mutations: increased fraction of transitional B cells and CD21loCD38lo chronically activated B cells, and often failure of class‐switching and progressive loss of B cells culminating in hypogammaglobulinemia [56, 57, 58].