Included in this study were fibroblasts derived from patients with two primary, mitochondrial diseases and two patients with genetic forms of Parkinson’s disease (PD) as follows; CI deficiency with fatal encephalomyopathy due to mutated NDUFAF4 (C6ORF66) (Saada et al., 2008), a CI assembly factor; and a relatively mild variant of cytochrome c oxidase (COX, mitochondrial respiratory chain complex IV) deficiency due to a mutated COX4I1, the common isoform of COX subunit 4, which presents with Fanconi anemia-like symptoms (Abu-Libdeh et al., 2017). Here, COX4I1 is linked to Parkinson disease.