X-linked hyper-IgM syndrome (XHIGM) is a combined immunodeficiency caused by mutations in CD40LG, which results in an inability to signal B cells to undergo isotype switching; thus, B cells produce only IgM and lead to elevated IgM levels and absent IgG, IgA, and IgE. This evidence concerns the gene CD40LG and X-linked hyper-IgM syndrome.