A decrease in Prevotellaceae in PD patients may be related to the reduction in mucin production defining an amplified intestinal permeability, known also as “leaky gut”, that can be associated with α-synuclein aggregates through the bacteria translocation and the production of bacterial products that induce inflammation [88] (i.e., gut-derived lipopolysaccharide (LPS) that can support the impairment of the blood–brain barrier [89]) and the reactive oxygen species production in the GI system thus starting the α-synuclein accumulation at the level of ENS [90]. The gene discussed is MUC5AC; the disease is Parkinson disease.