IgA deficiency and CVID can differentially affect members of the same kindred. Rarely, IgA deficiency can precede the development of CVID. Therefore, in some instances, the molecular mechanisms underlying CVID and IgA deficiency may be identical. TACI deficiency can cause IgA deficiency in some family members while others develop CVID. However, the molecular basis of IgA deficiency in most cases is largely unknown.