The brother who is homozygous (II.4) for the TNFRSF13B/TACI C104R mutation has the lowest IgG levels, and consistently generated fewer isotype switched and differentiated ASC in vitro, compared with other family members who are heterozygotes.20 The presence of concomitant mutations, such as the TCF3 T168fsX191 mutation seen in the proband, may explain the variable penetrance and expressivity of TNFRSF13B/TACI mutations in CVID. The gene discussed is TCF3; the disease is common variable immunodeficiency.