 The entire length of the vector SeV genome, including transgenes, has to be arranged in multiples of six nucleotides (the so-called "rule of six").Recombinant SeV variants has been constructed by introducing new genes and/or by deleting some viral genes such as F, M, and HN from the SeV genome, SeV constructs have also been created with a modified protease cleavage site in fusion protein (F).The SeV F protein is a type I membrane glycoprotein that is synthesized as an inactive precursor (F0) that must be activated by proteolytic cleavage at residue arginine-116.