We have identified a new mutant Cu/Zn superoxide dismutase (SOD1) deduced from the nucleotide sequences of peripheral blood lymphocyte mRNA from Japanese patients with familial amyotrophic lateral sclerosis (FALS). Sequence analysis of reverse transcriptase-initiated PCR amplified mRNA revealed a heterozygosity indicative of one normal allele and one variant allele with a T-->A transversion. This base change led to replacement of valine by glutamic acid at position 7 of 153-residue SOD1 molecule, and produced a new restriction site for Alu I in the exon 1. Restriction fragment length polymorphism analysis confirmed the linkage of this mutation with this type of FALS. Both enzymatic activity and protein of the SOD1 were reduced in red blood cells from the patient.