In about 20-25% of cases of familial amyotrophic lateral sclerosis (FALS) patients have mutations in the Cu/Zn superoxide dismutase (SOD1) gene. The mechanism through which the mutations in the SOD1 gene cause ALS still remain unknown. We performed pulse-chase experiments using a system for the transient expression of human SOD1 in COS7 cells to examine whether the Ala4Thr mutation, which we previously reported, decreases the stability of SOD1. The expression vector (pEF-BOS) carrying the wild-type or mutant (Ala4Thr) human SOD1 cDNA was transfected into COS7 cells, and transiently expressed human SOD1 was then metabolically radiolabeled. Half-lives of the wild-type and the Ala4Thr mutant SOD1 were determined to be 78 h and 18 h, respectively. These results suggest that the Ala4Thr mutation in SOD1 decreases the stability of SOD1 and that this instability may play an important role in the pathogenesis of the degeneration of motor neurons in FALS.