(A) MPII (PDB 4ON1), (B) FRA3 (PDB 3P24), (C) human ADAM-33 (PDB 1R55), (D) human MMP-12 (PDB 1Y93), (E) Protease C from Erwinia chrysanthemi (PDB 1K7I) and (F) Archaemetzincin from Methanopyrus kandleri (PDB 2X7M). Helices, cyan; β-sheets, magenta; loops, orange; prodomain (in MPII and FRA3) and a hydroxamate inhibitor (in ADAM-33), blue; Zn ion, grey sphere. The homologues were identified in the PDB database using FFAS03 software (http://ffas.sanfordburnham.org). Structural elements are labeled as in the respective original publications.