Bunina bodies, small eosinophilic intraneuronal inclusions, stain positive for cystatin C and are the only specific pathological hallmark of amyotrophic lateral sclerosis (ALS). We screened the cystatin C gene (CST3) for mutations in 57 sporadic ALS patients and 12 familial ALS cases that did not possess a SOD1 mutation. We detected the known polymorphism in exon 1, a G/A transition at +73, in both familial and sporadic ALS patients. However, the allelic and genotypic frequencies of the +73 G/A polymorphism did not differ between ALS patients and control samples. No other mutation was detected in the ALS patients. The results reported here indicate that there may not be a direct genetic link between cystatin C and ALS, and it may be that deficits occur in proteins that interact with cystatin C.