Definition: In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Input: To explore the critical genes associated with infectious bronchitis virus (IBV) virulence, we compared the genome sequences of virulent YN strain and its attenuated strain aYN. Accumulation of mutations in the S gene and the accessory gene 5a were observed, suggesting a potential role in the loss of viral pathogenicity. Two recombinant IBVs (rIBVs) with replacement of the S gene or 5a with corresponding regions from aYN were rescued to verify this speculation. Embryo mortality time/rate showed that rYN-S-aYN and rYN-5a-aYN had an attenuated phenotype in ovo. Replication assay in ovo and in vitro demonstrated the rIBVs had similar replication patterns with wild-type rIBV. Both rIBVs showed reduced mortality, tissue lesions and tissue virus titers in chicken. In conclusion, S gene and 5a accessory gene are responsible for the attenuation of virulent IBV. Insight into the genes responsible for virus attenuation will facilitate the development of future vaccines against IBV.

S gene and 5a accessory gene are responsible for the attenuation of virulent infectious bronchitis coronavirus