KRT5 and influenza: These include a lineage negative progenitor that subsequently expresses Trp63 and Krt5 (traditional basal cell markers) and is able to migrate into the alveoli to repair areas of severe damage after influenza injury [17,18], an α6β4 integrin-expressing cell type that proliferates in response to bleomycin-induced alveolar injury and proliferates and expands clonally in ex vivo culture [19], and a H2-K1 high distal airway epithelial population that can differentiate into alveolar structures [20].