Comprehensive clinical and molecular evaluations of patients with dyskeratosis congenita (DC) and their family members are being conducted to better understand the role of telomere biology defects in this disorder. DC is an inherited bone marrow failure syndrome characterized by abnormal nails, lacy skin pigmentation, oral leukoplakia, and significantly elevated risks of cancer. This project developed the only clinical diagnostic test of DC: telomere length measurement by flow FISH. DC is inherited in X-linked, autosomal dominant and autosomal recessive manners. NCI scientists discovered three novel causes of DC: (1) Germline mutations in TINF2: the first evidence that disruption of the shelterin protein protection complex causes human disease; (2) Mutations in WRAP53 were first proof that mislocalization of telomerase could cause disease; (3) Mutations in the helicase and telomere biology gene RTEL1 also cause DC. All patients with DC are also evaluated for mutations in all of the known DC genes, TINF2, DKC1, TERC, TERT, RTEL1, CTC1, WRAP53, NOP10, and NHP2. The DC workshop NCI hosted led to the creation of the only family support group for this rare disorder. NCI’s clinical and quantitative studies of cancer incidence, neuropsychiatric conditions, pulmonary complications, and genotype-phenotype correlations expanded prior notions of the DC phenotype. The DC Biospecimen Repository has catalyzed multiple collaborative studies with intramural and extramural scientists. For example, these biospecimens were the reagents used to identify that undifferentiated induced pluripotent stem cells accurately recapitulate features of a human stem cell disease (DC), and may serve as a cell-culture-based system for developing targeted therapeutics. Ongoing gene discovery efforts in mutation-negative families will yield important insights into telomere biology, and improvements in patient care.