S100A9 and Parkinson disease: Such elevated fecal calprotectin levels are evident in people with PD (Schwiertz et al., 2018; Mulak et al., 2019), and it is hypothesized that the amyloidogenic properties of calprotectin and S100A9 may contribute to fibril formation and enteric alpha-synuclein aggregation in response to gut inflammation (Mulak et al., 2019).