Criteria for the diagnosis of probable CVID are a marked decrease of IgG (at least 2 SD below the mean for age) and a marked decrease in at least one of the isotypes IgM or IgA and for possible CVID a marked decrease (at least 2 SD below the mean for age) in one of the major isotypes (IgM, IgG, and IgA) [11]. The gene discussed is CD40LG; the disease is common variable immunodeficiency.