That human RTEL1 is involved in telomere metabolism has been recently confirmed by the identification of RTEL1 mutations in patients with Hoyeraal-Hreidarsson syndrome (HHS), a severe form of dyskeratosis congenita characterized by short telomeres, developmental defects, bone marrow failure and immunodeficiency (7–11). The gene discussed is RTEL1; the disease is immunodeficiency disease.