SOD1 and Parkinson disease: Furthermore, Xu et al. suggested the oxidation of Cys111 in SOD1 to a sulfenic acid (−SOH) in CSF of a subset of sporadic ALS cases [103], and we also found that Cys111 was oxidized to a sulfonic acid (−SO3H) in CSF of a subset of ALS, PD, PSP, and AD cases [95].