Genetic mutations have been identified in the major motor neuron diseases,
including AZ,S, spinal muscular atrophy, bulbospinal muscular atrophy (Kenn
edy's disease), the hereditary spastic paraplegias, and rarer conditions su
ch as GM2 gangliosidosis (hexosaminidase A deficiency). These include mutat
ions in the SOD1 gene, deletions of the telomeric copy of the SMN gene, exp
ansions of the trinucleotide repeat region in the first exon of the androge
n receptor gene, other rare mutations, and diseases where linkage has been
established but the gene not identified. Identification of one of these gen
etic abnormalities will allow specific diagnosis in patients. Because cure
is not yet available, presymptomatic testing is seldom indicated; in such c
ases, careful counseling is appropriate.