Feline Infectious Peritonitis Virus (FIPV) is a coronavirus, an enveloped, positive-stranded RNA virus. S glycoprotein comprises S1 region and S2 region, S1 region attaches the virion to the cell membrane by interacting with feline ANPEP/aminopeptidase N, initiating the infection. Binding to the receptor probably induces conformational changes in the S glycoprotein unmasking the fusion peptide of S2 region and activating membranes fusion. S2 region belongs to the class I viral fusion protein.