The two surface glycoproteins of the mumps virus are the hemagglutinin-neuraminidase (HN) and Fusion proteins. These glycoproteins are essential for viral entry to host cells, and the spread of newly formed virions. The mumps fusion protein (F) is a 538-amino acid, class one fusion surface glycoprotein. It is responsible for the membrane fusion of virus and host cell. The un-cleaved protein has three hydrophobic regions: an amino-terminal signal peptide, an amino terminal region of F1 and the carboxyl-terminal membrane domain. This protein starts as a precursor molecule (F0), and is then cleaved into the active protein by the recognition of a R-X-L/R-R motif by a host endoprotease (furin). The F protein contains two disulfide-linked polypeptides (F1 and F2).