In contrast, loss-of-function mutations in PARN (PARN mutants) cause a severe form of DC called Hoyeraal-Hreidarsson syndrome, which causes abnormally short telomeres and congenital defects (38, 39, 40, 41) and idiopathic pulmonary fibrosis (IPF) (40). Here, PARN is linked to Hoyeraal-Hreidarsson syndrome.