We infected mice with the low-virulence TN/94-49 HMPV strain (44) and observed upregulation of type I IFN (IFN-β) and IFN-λ, but not type II IFN (IFN-γ), in mouse lung at an early time point (day 1) post-infection (Fig. 1A and B). Here, IFNB1 is linked to infection.