EXTL3 and immune system disorder: Skeletal dysplasias have been predominantly reported in association with humoral or combined immunodeficiencies (3), for instance with a large group of syndromic combined immunodeficiencies including cartilage-hair hypoplasia, Schimke immuno-osseous dysplasia, Roifman syndrome and the recently described skeletal dysplasia with neurological impairment, caused by mutations in the exostosin-like glycosyltransferase 3 (EXTL3) gene (1, 4–6).