IFNB1 and infection: By using IFNβ/YFP heterozygous MEFs, we showed that early after infection (<8 h post-infection [h.p.i.]), IFNβ gene expression was primarily monoallelic, while late in infection (8–16 h.p.i.), the majority of IFNβ-expressing cells were both IFNβ and YFP double-positive cells indicating that, as with human cells, a switch to biallelic expression also occurs in mouse cells (Figure S2B).