Hoyeraal-Hreidarsson syndrome (HHS) is a very rare X-linked recessive disorder considered to be a severe variant of dyskeratosis congenita characterized by intrauterine growth retardation, microcephaly, cerebellar hypoplasia, progressive combined immune deficiency and aplastic anemia. The disease is MONDO_0018045 (Hoyeraal-Hreidarsson syndrome). Also known as: progressive pancytopenia-immunodeficiency-cerebellar hypoplasia syndrome.