CD40 and inborn error of immunity: X-linked hyper-IgM syndrome (XHIGM; HIGM1; OMIM: 308230) is one type of primary immunodeficiency diseases (PIDs), resulting from defects in the CD40 ligand/CD40 signaling pathways leading to impairment of immunoglobulin isotype switching in B cells and characterized by recurrent infections in association with markedly decreased serum IgG, IgA, and IgE levels but normal or elevated serum IgM levels [1].