The X-linked variant of the Hyper-IgM syndrome. The affected individuals are virtually always male, because males only have one X chromosome, received from their mothers. Their mothers are not symptomatic, even though they are carriers of the allele, because the trait is recessive. Male offspring of these women have a 50% chance of inheriting their mother's mutant allele. The disease is MONDO_0010626 (hyper-IgM syndrome type 1). Also known as: HIGM1, HIGMX-1, Hyper IgM Syndromes, X-linked hyper-IgM syndrome, XHIGM, hyper-IgM syndrome due to CD40 ligand deficiency, hyper-IgM syndrome due to CD40L deficiency, hyper-IgM syndrome, X-linked.