Jain et al. described a small case series (n = 3) in which children with X‐linked hyper IgM syndrome (an immune deficiency disorder caused by mutations in the CD40 ligand gene) receiving treatment with recombinant CD40L (rCD40L) successfully mounted a KLH DTH response following study drug initiation, whereas no antibody responses to KLH were detected. This evidence concerns the gene CD40LG and inborn error of immunity.