Regarding the immunological characterization, according to ESID criteria [4] and Janssen et al.’s definition [7,46], “truly” selective SIgMD has been defined as repeatedly decreased serum IgM levels, normal levels of serum IgG, IgA, IgG subclasses, and normal vaccination responses and the exclusion of T cell defects through the absence of clinical signs suggesting a T cell defect. The gene discussed is CD79A; the disease is selective IgM deficiency.