PET100 and Leigh syndrome: Pathogenic variants in a number of the assembly factors necessary for the formation of a functional COX enzyme have been reported.9,14, 15, 16, 17, 18, 19, 20 Recently, a founder mutation in a highly conserved COX assembly factor PET100 has been identified in 10 Lebanese individuals with isolated COX deficiency who present with Leigh syndrome and seizures.24