The clinical criteria sufficient for a probable diagnosis of XLA include a male with less than 2% CD19+ B cells in whom other causes of hypogammaglobulinemia have been excluded, who has at least one of the following: onset of recurrent bacterial infections in the first 5 years of life, serum IgG, IgM and IgA that are more than 2 SD below normal for age, and poor responses to vaccines. Here, CD79A is linked to agammaglobulinemia.