Mutations in the superoxide dismuate-1 (SOD-1) gene underlie approximately 20% of all familial forms of amyotrophic lateral sclerosis (FALS-1), and to date over 100 different mutations have been reported.1 Although FALS-1 is regarded as clinically and pathologically indistinguishable from sporadic ALS, marked variations in penetrance, age and site of disease onset, rate of disease progression and survival have been reported with different SOD-1 mutations.2 The pathophysiological mechanisms underlying these marked variations in SOD-1 genotype expression remain elusive, and resolution of this could be of therapeutic significance.

In the paper by Blain and colleagues3 (see page 843), diffusion tensor …