In the most severe conditions such as MHC class II deficiency [299] Wiskott–Aldrich syndrome, DOCK8 deficiency [300], CD40 ligand deficiency [301], CD27/70 deficiency [302] and a growing number of related diseases allo-HCT has to be strongly considered early in life, particularly if an HLA-matched donor is available. This evidence concerns the gene CD40LG and hyperinsulinemic hypoglycemia, familial, 4.