Evidence of cortical hyperexcitability in asymptomatic carriers of genetic mutations who are “at risk” of ALS is restricted to a very limited finding of reduced ligand [11C]‐flumazenil binding in two individuals with the D90A SOD1 mutation [Turner et al., 2005a], and three SOD1 mutation carriers who demonstrated reduced or absent intra‐cortical inhibition within three months of symptom onset [Vucic et al., 2008]. This evidence concerns the gene SOD1 and amyotrophic lateral sclerosis.