P4HB and amyotrophic lateral sclerosis: As previously shown for Parkinson's and Alzheimer's brain tissues [50], posttranslational modification of PDI by S-nitrosylation of the critical active site cysteine residues, leading to the inhibition of PDI enzymatic activity, has been observed also in spinal cord tissues of sporadic ALS patients and in transgenic SOD1G93A mice.