Its sequence contains a total of eight cysteine (C) residues (C21, C251, C280, C321, C323, C334, C384, and C470). Five residues (C280, C321, C323, C334, and C384) are clustered in the C-terminal half of the putative extracellular domain (ECD). The remaining three cysteine residues are equally distributed in the N-terminal transmembrane domain (C21), N-terminal half of the ECD (C251), and the C-terminal domain (C470). With the exception of C21, the remaining seven residues are highly conserved in other species including mouse, hamster, rabbit, pig, cow and human. Among the three artificially created cysteine residues, two (G453C and Y462C) were located in the C-terminal transmembrane domain, while the third one (V3C) was in the N-terminal domain.