TLR4 and Parkinson disease: Although pre-existing polymorphisms in TLR4 have been linked to PD, it is unclear if other genes that influence the signalling of TLR4 (e.g., PPARγ and Tollip) or regulate exposure to various pro-inflammatory microbial metabolites (e.g., tight-junction proteins which mediate intestinal permeability) also influence PD risk or progression.