For example, in a large Japanese family with X- linked agammaglobulinemia (XLA), a patient (referred to as P2) harboring a splice donor mutation (IVS11+3G>T) in BTK resulting in the skipping of exon 11 still had a leaky expression of normal size BTK transcripts resulting in residual BTK protein expression on B cells and peripheral blood mononuclear cells (PBMCs) (4). This evidence concerns the gene BTK and isolated agammaglobulinemia.