Indeed, increased levels of TGF-β, MAPK, and/or MSTN have been associated with spinal muscular atrophy and Kennedy disease [101,102], and inhibition of MSTN improves familial amyotrophic lateral sclerosis (ALS) [82,87], but this review focuses on altered signaling in the pathogenesis of Marfan syndrome (MFS) and the muscular dystrophies. The gene discussed is MSTN; the disease is spinal muscular atrophy.