Infection with VV/ΔHA and VVΔE3L/NS1 viruses in BHK21 cells pretreated with different doses of IFN-β (from 0 to 1000 U/ml) indicated that both viruses exhibit similar resistance to IFN inhibition, whereas VVΔE3L is highly sensitive (data not shown). This evidence concerns the gene IFNB1 and infection.