ER-60 protease contains two CGHC motifs that appear to include an active site cysteine residue(s). Its proteolytic activity was lost with a double mutation of the C-terminal cysteines of the two motifs to alanine, but not with a single mutation of the C-terminal cysteine of either of the motifs to alanine. This suggests that these C-terminal cysteines independently constitute the catalytic active site. A mutation of both histidine residues in the two CGHC motifs to serine did not abolish the proteolytic activity, suggesting these histidine residues in the CGHC motifs do not constitute the catalytic dyad of ER-60 protease.