USB1 and poikiloderma with neutropenia: Mutations in the USB1 (C16orf57) (U6 snRNA biogenesis 1, OMIM #613276) gene give rise to autosomal‐recessive Clericuzio‐type poikiloderma with neutropenia (PN, OMIM #604173), characterized by early‐onset poikiloderma (appearing in the first year of life), nail abnormalities, palmoplantar hyperkeratosis, skeletal defects as well as noncycling neutropenia and recurrent sinopulmonary infections (Colombo et al.