The spinal muscular atrophies are a group of mostly inherited disorders 
selectively affecting the lower motor neuron. There is a wide degree of clinical 
and genetic heterogeneity that must be taken into account when giving prognostic 
information. Autosomal recessive childhood proximal SMA is the commonest form 
and is due to mutations in a gene encoding a novel protein, SMN, that appears to 
play a critical role in RNA metabolism but has also been shown to interact with 
actin-binding proteins and mediators of programmed cell death. The 
identification of the genetic basis of SMA has resulted in advances for prenatal 
diagnosis and in new insights into motor neuron biology. The chromosomal 
location of two of the rarer dominant forms of SMA has been found. 
Identification of the molecular pathophysiology of lower motor neuron syndromes 
can be expected to aid in the development of therapy for these disabling 
disorders.