Two independent studies of gene-targeted mice with TCF3 haploinsufficiency have shown reduced numbers of B cells and impaired lymphoid cell development.23, 38 Similarly, reduced expression of TCF3/E2A has been implicated in equine CVID.39 There is thus strong support from human, murine and equine studies for the pathogenicity of the TCF3 T168fsX191 mutation in our family. The gene discussed is TCF3; the disease is common variable immunodeficiency.