Serine protease HTRA2, also known as high temperature requirement protein A2, Omi stress-regulated endoprotease, Serine protease 25, Serine proteinase OMI and HTRA2, is a single-pass membrane protein which belongs to the peptidase S1B family. HTRA2 contains one PDZ (DHR) domain. HTRA2 is a serine protease that shows proteolytic activity against a non-specific substrate beta-casein. It promotes or induces cell death either by direct binding to and inhibition of BIRC proteins (also called inhibitor of apoptosis proteins, IAPs), leading to an increase in caspase activity, or by a BIRC inhibition-independent, caspase-independent and serine protease activity-dependent mechanism. HTRA2 cleaves THAP5 and promotes its degradation during apoptosis. Isoform 2 of HTRA2 seems to be proteolytically inactive. Defects in HTRA2 are the cause of Parkinson disease type 13 (PARK13) which is a complex neurodegenerative disorder characterized by bradykinesia, resting tremor, muscular rigidity and postural instability, as well as by a clinically significant response to treatment with levodopa.