DDIT3 and pulmonary fibrosis: However, Chop(−/−) ko mice were not protected from lung fibrosis in response to single-dose intratracheal bleomycin; but the explanation for this phenomenon is very different from that by Ayaub et al. Burman and colleagues observed that single intratracheal bleomycin was not sufficient to induce robust Chop upregulation in wild-type mice thus offering a potential reason why Chop deletion was not protective in this model [26].