Many lines of evidence show that mutant SOD1 in familial amyotrophic lateral sclerosis (ALS) gains a novel toxic property, causing neuronal cell death.1 A simple and attractive therapeutic approach, therefore, is to inhibit the expression of mutant SOD1. Small interfering RNA (siRNA) can strongly suppress expression of the target gene and become a powerful tool in gene therapy for dominantly inherited neurodegenerative diseases.2 Recently, we reported that the onset of ALS symptoms was extremely delayed in SOD1G93A transgenic (Tg) mice when crossed with the Tg mouse overexpressing siRNA to SOD1.3 Here, we furthermore found that this transgenic siRNA could markedly slow the progression of the disease as well.