BAFF receptor deficiency due to TNFRSF13C gene mutation is one the IEI showing a CVID phenotype (defined as an age-specific reduction in the serum concentrations of IgG in combination with low levels of IgA and/or IgM and a poor or absent response to immunizations and/or absent isohemagglutinins and/or low switched B cells (<70 percent of age-related normal value) and the absence of profound T cell immunodeficiency and absence of any other defined immunodeficient state [89,90]) characterized by low IgG and IgM serum levels with a variable clinical expression [3]. This evidence concerns the gene TNFRSF13C and hyperinsulinemic hypoglycemia, familial, 4.