SOD1 and amyotrophic lateral sclerosis: In examining the frequency of mutations in positively charged residues of SOD1 (Fig 1, arrows), we noted that only one of 11 Lys (K3 –red arrow) and one of four Arg (R115 –red arrow) residues are known mutation sites in ALS {http://alsod.iop.kcl.ac.uk/default.aspx} (Fig 1, red font indicates positions of known ALS mutations).