TK2 and inborn mitochondrial metabolism disorder: Interestingly, elevated circulating GDF15 levels were reported in patients with mitochondrial diseases, including PMM; thus, it has been put forward as a useful biomarker for mitochondrial disorders [23, 24] (Supporting Information: Reference S41), as well as a potential therapeutic biomarker for myopathy in patients with thymidine kinase 2 (TK2) deficiency [1, 25].