gp40 -is a transmembrane glycoprotein responsible for viral membrane fusion with the host cell membrane. The gp40 subunit contains different functional regions in its ectodomain: the fusion peptide (FP), heptad repeat 1 (HR1), HR2, and the transmembrane region. This protein is very similar to gp41 from HIV. The sequence of gp40 contains four functional regions, as follows: the N-terminal fusion peptide (which is thought to insert into the host cell membrane) is followed by an ectodomain, a transmembrane region and a cytoplasmic domain.

Recombinant antigen for the diagnosis of Feline immunodeficiency syndrome (feline AIDS) caused by Feline immunodeficiency virus (FIV), an animal infectious disease (mainly cats).