PET117 is a mitochondrial chaperone protein essential for cytochrome c oxidase (complex IV) assembly and function. Primary Function: PET117 acts as an assembly factor for complex IV, the terminal enzyme of the electron transport chain that catalyzes electron transfer from cytochrome c to oxygen, enabling ATP production 1. Mechanism: PET117 stabilizes TACO1, a translational activator of mitochondria-encoded cytochrome c oxidase 1 (COX1), preventing its ubiquitin-mediated degradation and thereby upregulating COX1 synthesis 1. Additionally, PET117 functions within a module-based assembly pathway, coordinating with chaperones PET100 and MR-1S to integrate mtDNA-encoded core subunits with nucleus-encoded subunits 2. Disease Relevance: Homozygous nonsense mutations in PET117 cause mitochondrial complex IV deficiency, manifesting as neurodevelopmental regression and medulla oblongata lesions 3. PET117 deficiency reduces oxygen consumption and impairs mitochondrial function 1. Clinical Significance: PET117 mutations represent a novel genetic cause of nuclear-encoded mitochondrial disease. Lentiviral complementation with wild-type PET117 restores complex IV activity in patient fibroblasts, confirming the causal relationship and suggesting potential therapeutic targets 3.