Amyotrophic lateral sclerosis (ALS) is found in a familial form in around 5-10% 
of cases. Of these familial cases around 20% are associated with mutations of 
SOD-1. The genetic basis of the disease in the remaining familial cases, and 
genetic risk factors in sporadic cases, are unknown. Recently, the common forms 
of spinal muscular atrophy (SMA) have been associated with mutations of the SMN 
and NAIP genes on chromosome 5, in the region q11.2-13.3. Some patients with 
both familial and sporadic motor neuron disease show only lower motor neuron 
signs, in common with SMA patients, and families containing individuals with 
phenotypes of both childhood SMA and adult motor neuron disease have been 
reported. We therefore examined the SMA locus as a candidate for ALS, in 54 
patients with sporadic motor neuron disease, and 10 single-generation familial 
patients (with no evidence of SOD-1 mutations), and in a single patient with 
Brown-Vialetto-Van Laere syndrome. No mutations of the SMN or NAIP genes were 
detected. The difficulties of classification of lower motor neuron presentations 
of motor neuron diseases are discussed. The demonstration that mutations 
diagnostic of SMA are not found in ALS patients helps distinguish these 
conditions.