Several monogenetic defects including homo/heterozygous mutations in ICOS (inducible costimulator of activated T cells), TACI (transmembrane activator and calcium-modulator and cyclophilin ligand interactor), BAFF-R (B cell activating factor receptor), CD19, CD81, and Msh5 were found in CVID patients, but genetics of CVID are complicated by unaffected relatives that carry the same mutations showing that the genotype–phenotype correlation can be variable (6). The gene discussed is CD19; the disease is common variable immunodeficiency.