Two of the best examples of a disorder associated with a pseudouridylation defect are X-linked Dyskeratosis Congenita (X-DC) and its more severe form, Hoyeraal-Hreidarsson syndrome; these are two rare inherited syndromes caused by mutations in DKC1, the gene encoding for the pseudouridine synthase dyskerin. This evidence concerns the gene DKC1 and Hoyeraal-Hreidarsson syndrome.