SPP1 and amyotrophic lateral sclerosis: OPN levels may be elevated in the plasma, urine, cerebrospinal fluid (CSF), and brain of subjects with neurodegenerative disease, including AD [3,95,122,123,124,125], multiple sclerosis (MS) [116,122,126], Parkinson’s disease [116,122], amyotrophic lateral sclerosis [3,127], HIV-associated neurocognitive disorder [4,128], and mild cognitive impairment (MCI) [129].