FURIN and infection: In contrast the virulence of NDV is known to be associated with different amino acid sequence surrounding the post-translational cleavage site of the F0 protein, which is needed to be cleaved by the host cell proteases for induction of infection and not dependent upon trypsin for cleavage, but can be cleaved by furin which recognizes the multibasic cleavage site.13 Most viruses that are virulent for chickens have the amino acid sequence 112-R/KR-Q-K/R-R-116 at the C-terminus of the F2 protein and F (phenylalanine) at residue 117, the N-terminus of the F1 protein.