Bleomycin is an anti-cancer drug, which induces lung injury and fibrosis [60] . As shown in Figures 9A and 9B , treatment of adult BRE-eGFP animals by intra-tracheal administration of bleomycin resulted in substantial reactivation of the eGFP reporter in type-II pneumocytes (Pro-SpC pos ). During the first days of treatment the eGFP pos cells were primarily cuboidal SpC pos cells. From day-4 onwards both cuboidal and squamous eGFP pos cells were found in the alveolar regions. Interestingly, the squamous eGFP pos cells were T1a-expressing type-I pneumocytes and were always localized in the borders between affected and apparently normal tissue ( Figure 9C ). We postulate that the cuboidal eGFP pos cells observed in the early stages of the bleomycin induced response represent a reparatory progenitor population that expands and differentiates into the eGFP pos -T1a low cells observed in the later stages to repair the damaged alveolar epithelium. Further studies will be required, however, to validate this hypothesis.