X-linked hyper-IgM Syndrome (XHIGM) is caused by a mutation of CD40 ligand (CD40L), which is normally expressed on activated CD4+ T cells and is responsible for immunoglobulin class switching. A 7-year-old boy with recurrent sino-pulmonary infections since the age of 3 months had normal CD3+, CD4+, CD8+T lymphocytes, and CD19+B lymphocytes and NK cells, but significantly elevated IgM and extremely decreased IgG and IgA. Sequencing of genomic DNA revealed that the patient had a 34 base deletion in intron 3 and exon 4 of CD40L(g.8172_8205del34bp), which lead to the entire deletion of exon 4 in cDNA (c.347_409del63bp, i.e.,exon 4 skipping) and an in-frame deletion of 21 amino acids in CD40L protein. Moreover, the patient had negligible CD40L expression on activated CD3+CD8-T lymphocytes. His mother and sister were carriers of the CD40L mutation. Our studies demonstrated a novel mutation in CD40L, which, to our knowledge, has not been reported previously.