RESULTS: In this study, wefind 18 SNPsin BTK gene, including three novel SNPs. The mutated site X: 101360588, c.756G> A (p.Trp252Ter) located in SH3 domain may greatly weaken the binding capacity of SH3BP5. The exon 8, where this site is located in, was sequenced by Sanger sequencing. The flow cytometry of the blood shows that the expression of BTK protein in mononuclear cells of female carriers are different from the normal. The novel mutated site X: 101360588, c.756G> A (p.Trp252Ter) can indeed cause BTK protein product detected, therefore causing obstacles to the development of B lymphocytes and led to XLA.