Atf4 overexpression in adult muscle was shown to reduce myofibres size and some of its downstream target genes may interfere with muscle development.27 In particular Fgf21, a sensitive marker of muscle mitochondrial diseases,28, 44 was strongly upregulated in skeletal muscles and in blood of Drp/MC mice. The gene discussed is FGF21; the disease is inborn mitochondrial metabolism disorder.