s at the S1/S2 cleavage site [53], the S1 subunit is responsible for receptor binding to the host cells [54]. Whether the S1 gene has an effect on the virulence of the virus is unknown. In this study, the recombinant strain, rH120-S1/YZ, expressed the S1 gene of the QX-like strain, which retains the S1/S2 cleavage site of H120, maintained the abi