The HN protein of NDV is a multifunctional protein. It possesses both the receptor recognition and neuraminidase (NA) activities associated with the virus. It recognizes sialic acid-containing receptors on cell surfaces; it promotes the fusion activity of the F protein, thereby allowing the virus to penetrate the cell surface; and it acts as an NA by removing the sialic acid from progeny virus particles to prevent self-agglutination of progeny virus. Thus, the HN protein plays an important role in viral infection. Although the functions of the HN protein in NDV infection have been well studied, its role in NDV pathogenesis is not known at present. It has recently been shown that the cleavability of the F protein alone does not convert an otherwise nonpathogenic strain into a highly virulent pathotype.