To determine whether RTEL1 mutations are a significant cause of HHS, we extended the screen of RTEL1 to a larger group of individuals. On the basis of the clinical definition of HHS and the presentation of the initial family, we selected 23 additional index cases with global BM failure and cerebellar hypoplasia from our DCR (Table S2). We scanned for mutations by denaturing high-performance liquid chromatography on a Transgenomic Wave DNA-fragment analysis system (Transgenomic, Glasgow, UK). Any fragments showing abnormal elution patterns were reamplified, and variants were confirmed by Sanger sequencing. We considered nonsynonymous, splice-site, or loss-of-function variants absent from the 1000 Genomes Project as potentially disease-causing mutations.