Familial amyotrophic lateral sclerosis (FALS) accounts for about 5% of cases of the neurodegenerative disorder ALS. At least 100 Cu/Zn superoxide dismutase (SOD1) genetic mutations have been associated with FALS. We identified a FALS family in China with an atypical clinical phenotype. To investigate the SOD1 gene mutations in this family, five exons of the SOD1 gene from each living patient were amplified by PCR and screened by SSCP and direct DNA sequencing. SSCP analysis demonstrated a mutation in exon 2 of SOD1, and DNA sequencing demonstrated the presence of an insertion mutation in exon 2 that has not been reported previously. The mutant SOD1 gene encodes a truncated protein of 35 amino acid residues compared to the normal SOD1 protein of 153 amino acids. In conclusion, The SOD1 exon 2 mutation is likely to be the etiological factor of ALS in this family.