Hyper-immunoglobulin M syndrome (HIGM) is a rare (incidence, 1 in 100,000 births) primary immunodeficiency[4,5], in which defective B cell isotype switching leads to a phenotype characterized by elevated or normal levels of serum IgM, and low levels of serum IgG, IgA and IgE[5]. Here, CD40LG is linked to hyper-IgM syndrome.