A targeting construct was designed to insert a loxP-flanked transcriptional STOP cassette (Puro-4x polyA) between exons 2 and 3, and an FRT-flanked neomycin (neo) resistance cassette downstream of exon 7 of the gene. A mutation was introduced in amino acid 267 in exon 6, corresponding to human amino acid 280, resulting in a lysine-to-glutamate mutation, (K267E) commonly found in patients with dyskeratosis congenital (DC). Mutant mice were bred with Tg(ACTFLPe)9205Dym mice (on a C57BL/6 congenic background (N10)) to remove the neo selection cassette.