We found increased transcript levels of Fgf21 in COX10 KO muscle (Fig 4A) and chronically elevated levels of circulating FGF21 protein commencing from the earliest stage of myopathy (9‐, 4‐, and 5‐fold at 50, 100, and 200 days, respectively, Fig 4J). Here, COX10 is linked to myopathy.