Complex sentence: The most common among muscular atrophies is spinal muscular atrophy (SMA), caused by a mutation of the SMN1 gene on chromosome 5 which manifests over a wide range of severity affecting infants through adults.
Split into simpler sentences: The most common among muscular atrophies is spinal muscular atrophy (SMA), caused by a mutation of the SMN1 gene on chromosome 5. It manifests over a wide range of severity affecting infants through adults.