Consistently, the mRNA level of isg15, a typical ISG gene, was also significantly upregulated and reached the peak at 1 day in the liver, spleen and kidney (Figure 3I,K) or 3 days (in gill, Figure 3L) post GCRV infection (dpi) and then returned to the initial level, indicating that the challenge experiment successfully induced an IFN response. This evidence concerns the gene ISG15 and infection.