TPH1 and fatty liver disease: As gut-specific tryptophan hydroxylase 1 (Tph1) knockout mice and liver-specific Htr2a knockout mice are resistant to high-fat diet (HFD)-induced hepatic steatosis, the gut TPH1-liver-HTR2A axis is a promising drug target for NAFLD (Crane et al., 2015; Choi et al., 2018).