XIAP and Parkinson disease: Up until now, the research on HtrA2 and PD essentially focused on the link between HtrA2 and apoptosis during mitochondrial stress, since it was reported that, upon cytosolic retention, HtrA2 can act as a pro-apoptotic signal; indeed, cytosolic HtrA2 can cleave Inhibitor of Apoptosis Proteins (IAPs), such as c-IAP1 and XIAP, thereby promoting apoptosis37,38.