IFITM3 and infection: Although we cannot formally exclude the possibility that IFITM3 may influence viral replication in an unidentified cell type in vivo, the observation that viral load after 4 days of infection was comparable in WT and Ifitm3–/– mice following NK cell depletion or after challenge with Δm157 MCMV strongly suggests that impaired NK cell responses were primarily responsible for the elevated viral replication in Ifitm3–/– mice.