A definitive diagnosis of XLA is possible, based on the European Society for Immunodeficiencies (ESID) criteria, in a male presenting with CD19+ B cells < 2% and a confirmed mutation in the BTK gene and/or an absent BTK protein expression, and/or a male family member of maternal lineage with CD19+ B cells <2% (8). The gene discussed is CD19; the disease is Immunodeficiency.