Mutations in the CD40 ligand (CD40L) gene (<i>CD40LG</i>) lead to X-linked hyper-IgM syndrome (X-HIGM), which is a primary immunodeficiency (PID) characterized by decreased serum levels of IgG and IgA and normal or elevated IgM levels. Here, CD40LG is linked to X-linked hyper-IgM syndrome.