CVID is defined by markedly reduced serum concentrations of immunoglobulin G (IgG), usually below 400 mg/dL, along with at least one of the following: poor or absent response to immunization, low levels of immunoglobulin A (IgA) and/or immunoglobulin M (IgM), or the absence of other immunodeficiencies [2-4]. The gene discussed is CD40LG; the disease is common variable immunodeficiency.