Mutations in genes encoding TACI (TNFRSF13B), ICOS (ICOS), CD19 (CD19), and, most recently, B cell activating factor receptor (BAFF-R; TNFRSF13C) (Warnatz et al, 2009) all involved in the process of B cell activation, have been described in CVID. Here, ICOS is linked to common variable immunodeficiency.