RC complex deficiency is more severe in muscle cells. Immunoblot of RC complex and YARS2 levels in YARS2 patient fibroblasts and MyoD-forced myotubes (A). C = control, P2 = [p.Phe52Leu];[p.Phe52Leu] YARS2, P6 = [p.Gly191Asp];[p.Arg360X] YARS2. Complex IV activity was measured relative to total protein (B) and citrate synthase activity (C) in control (C) and patients (P2 & P6) fibroblasts (Fib) and MyoD-forced myotubes (Myo). Experiments were performed at least twice, in triplicate each time. Control fibroblasts were normalised to 100%. Values are the mean ± SD.