Telomeres are complex nucleoprotein structures that protect chromosome ends from cellular exonucleases and nonhomologous-end joining and distinguish them from double-stranded breaks, which can occur elsewhere in the genome. This protection is achieved not only by the involvement of proteins of the shelterin complex13 but also by the creation of a lasso-like configuration called a T-loop by the 3′ end of telomeres.14 In this structure, a 3′ single-stranded overhang invades the double-stranded telomeric region and displaces the identical strand of DNA to form a displacement-loop (D-loop) at the base of the T-loop. For DNA replication to occur, these loops need to be resolved while maintaining telomere integrity. It is also important that these telomeric-strand invasions are distinguished from the preliminary step of a homologous recombination event during double-strand break repair.13 Conditional mouse knockout studies have demonstrated that RTEL1 is essential for telomere maintenance in terms of both length and stability.10 To determine whether this observation holds true in individuals with RTEL1 mutations, we used quantitative PCR to measure telomere length in samples for which we had DNA of sufficient quality (n = 5). A monochrome multiplex quantitative PCR method adapted for use on a LightCycler 480 real-time thermocycler (Roche, Little Chalfont, UK) was used as described previously.15,16 The telomere lengths were significantly shorter in the group with biallelic RTEL1 mutations than in controls (n = 71) (p = 0.0003, Mann-Whitney U, Figure 2A).