To determine whether the amino acid responsible for the ts phenotype of M41-R in Nsp 14 had any effect on the IFN-I-mediated immune response, which has been shown previously to be modulated during IBV infection (35, 36), the upregulation of IFN-α and IFN-β, as determined by quantitative real-time PCR (qRT-PCR), was assessed during infection of primary CK cells with either M41-K, M41-R, or the parental wild-type virus M41-CK at both 37 and 41°C (Fig. 5). This evidence concerns the gene IFNB1 and infection.