 Therefore, the strains that were isolated from animals a few decades ago and went through multiple passages in eggs are less virulent for mice in comparison with the strains that are fresh field isolates.Defective interfering (DI) genomes or defective viral genomes (DVGs) are replication defective viral RNA products generated during viral infections by many types of viruses, including SeV.A single amino acid substitution in a nucleoprotein (NP) causes an increased production rate of DI genomes in the SeV Cantell strain, which is known for its particularly strong induction of interferon beta (IFN-β) during viral infection.