HtrA2/Omi is a mammalian serine protease at high temperatures and a chaperone at low temperature. The full-length HtrA2 is synthesized as a precursor protein and then targeted to the mitochondria where it is matured by the removal of N-terminal 133 residues. Mature HtrA2 consists of a putative transmembrane domain; an inhibitor of apoptosis protein (IAP)-binding motif; a single C-terminal PDZ domain that mediates protein-protein interactions. Recently, HtrA2 has known to contribute both to caspase-dependent and caspase-independent cell death.