FGF21 and neurodegenerative disease: By analogy with the amyloid-tau-neurodegeneration (A/T/N) classification in neurodegenerative diseases, a similar approach could be considered for mitochondriopathies, including biomarkers for metabolic disturbance (FGF21), apoptosis/cellular stress (GDF15) and lysosomal dysfunction (CHIT1) [38].