CD40LG and immunodeficiency disease: X-linked hyper-IgM syndrome (XHIGM) caused by the CD40 ligand (CD40L) mutation is a rare primary immunodeficiency condition that decreases immunity to opportunistic infections because there are defects in neutrophils, complement proteins, macrophages, and lymphocyte subsets.[1,2] A missed or delayed diagnosis of XHIGM is widespread and concerning, especially in patients without a relevant family or medical history.