Dibasic or multibasic cleavage motifs enable intracellular fusion protein cleavage by furin or related proteases during transit through the trans-Golgi network, and influenza virus and NDV with multibasic cleavage motifs have been shown to have a dramatically expanded range of cells that cleave their fusion proteins, resulting in more severe infections in domestic poultry and other natural hosts including waterfowl (33, 66, 67, 69). This evidence concerns the gene FURIN and infection.