Homology models of the catalytic domains of polyserase-3 and β-II tryptase. The structural modeling of Spd-1 and Spd-2 reveals the high degree of similarity with the serine protease β-II tryptase. The twelve loops (six in each serine protease module) potentially involved in the dimerization of polyserase-3 as well as the 6 loops involved in β-II tryptase tetramerization are represented using the color code previously described by Sommerhoff et al. 1999 (33). The molecules are oriented towards the active site and the three residues that compose the catalytic triad of each serine protease are indicated. The backbone and side chains of the disulfide bonds are represented in CPK color scheme.