Unlike previous studies of the pathogenic MNV1 isolate (41), infection at an m.o.i. of 1 TCID50/cell by both WT and mutant F1/F2 MNV3 did not stimulate a detectable IFN-β response as measured by quantitative PCR for mRNA transcripts at either 8 or 24 h time points in either RAW264.7 cells or primary BMDMs (data not shown). The gene discussed is IFNB1; the disease is infection.