A polybasic cleavage site explains a virus that is highly-pathogenic for humans while it is low-pathogenic for other species. For example, using reverse genetic tools, an avian paramyxovirus type 7 (APMV-7) was developed by mutating the fusion (F) protein cleavage site [4]. The constructed APMV-7 showed furin cleavage and increased replication and syncytium formation in cell cultures. However, chicken exposed to the virus did not exhibit infection.