A central feature of DC and HHS is defective maintenance of telomeres and mutations in one of 11 genes controlling telomere homeostasis, including DKC1, TERT, RTEL1, PARN, TINF2 (linked to both DC and HHS), TERC, WRAP53, NOP10, NHP2, CTC1 (linked only to DC), and ACD (TPP1 protein, linked only to HHS), have been detected in ~70% of cases with DC and 50% with HHS3–18. The gene discussed is RTEL1; the disease is dyskeratosis congenita.