Spinal muscular atrophy
Spinal muscular atrophy (SMA) is an autosomal recessive disease caused by a genetic defect in the SMN1 gene that codes SMN, a protein widely expressed in all eukaryotic cells. SMN1 is apparently selectively necessary for survival of motor neurons, as diminished abundance of the protein results in death of neuronal cells in the anterior horn of the spinal cord and subsequent system-wide muscle wasting (atrophy).
Spinal muscular atrophy manifests in various degrees of severity which all have in common general muscle wasting and mobility impairment. Other body systems may be affected as well, particularly in early-onset forms. SMA is the most common genetic cause of infant death.
The term spinal muscular atrophy is used as both a specific term for the genetic disorder caused by deficient SMN, and a general label for a larger number of rare disorders having in common a genetic cause and slow progression of weakness without sensory impairment caused by disease of motor neurons in the spinal cord and brainstem –
(Note: The information above is courtesy of Wikipedia)