For instance, AAV2/9-delivered survival motor neuron overexpression results in sensorimotor toxicity and reverses the initial benefits for spinal muscular atrophy therapy13, and AAV-delivered fibroblast growth factor 21 (FGF21) results in bone loss owing to constitutive gene expression at physiological concentrations14. This evidence concerns the gene FGF21 and spinal muscular atrophy.