Hoyeraal–Hreidasson syndrome) is a very rare multisystem X-linked recessive disorder characterized by excessively short telomeres and is considered a severe form of dyskeratosis congenita. Being an X-linked disorder, Hoyeraal–Hreidasson syndrome primarily affects males. Patients typically present in early childhood with cerebellar hypoplasia, immunodeficiency, progressive bone marrow failure, and intrauterine growth restriction. The primary cause of death in Hoyeraal–Hreidasson syndrome is bone marrow failure, but mortality from cancer and pulmonary fibrosis is also significant.