Basal cells expressed a modest number of ISGs upon infection, specifically genes of the IFITM family, IFI27 and IFI6 and the negative regulator of IFN-I signalling, ISG15. Consistent with this finding, gene-set enrichment analysis (GSEA) identified upregulation of IFN alpha/gamma responses in infected basal cell populations but not in other cell types (Fig. 3c). This evidence concerns the gene ISG15 and infection.