Selective IgM deficiency (SIgMD) is characterized by serum IgM levels below two standard deviations (SDs) of the mean for age, with normal IgG and IgA levels, and normal T-cell numbers and function, after excluding secondary causes such as infections and protein-losing conditions [6]. The prevalence of SIgMD is low in the general population, with community health screening surveys indicating a prevalence of approximately 0.03% for “complete” SIgMD. Here, CD40LG is linked to selective IgM deficiency.