PET100 is a mitochondrial chaperone protein essential for cytochrome c oxidase (complex IV) assembly and maturation. The protein localizes to the mitochondrial inner membrane where it functions as part of a module-based assembly process for complex IV biogenesis 1. PET100 works in conjunction with other highly conserved chaperones including PET117 and the newly identified MR-1S to facilitate proper complex IV assembly in higher eukaryotes 1. The protein forms a ~300 kDa subcomplex with complex IV subunits during the assembly process 2. Mutations in PET100 cause isolated mitochondrial complex IV deficiency, resulting in severe clinical phenotypes including Leigh syndrome and fatal infantile lactic acidosis 23. A founder mutation (c.3G>C) abolishing the initiation codon has been identified in Lebanese populations, causing complex IV deficiency with seizures as a prominent feature 2. Truncating variants such as p.(Gln48*) result in complete loss of enzyme activity and holocomplex assembly 3. The gene has also been associated with cancer prognosis, with PET100 expression levels correlating with mitochondrial activity in mast cells and serving as a prognostic biomarker in ovarian cancer 4.