We recruited individuals who were classified as having DC on the basis of previously published clinical criteria7 to the Dyskeratosis Congenita Registry (DCR) in London. They were also included if they had at least four of six of the most commonly recognized HHS-associated features (intrauterine growth restriction, developmental delay, microcephaly, cerebellar hypoplasia, immunodeficiency, and BM failure). All samples were obtained with informed consent and the approval of our local ethics committee.