5-HT3RAs such as tropisetron can reduce hepatic lesions of obesity-associated fatty liver disease in mice by reducing portal vein plasma endotoxin levels, attenuating the increased MyD88 and tumor necrosis factor-α mRNA expression in the liver, and increasing tight junction proteins in the duodenum (Haub et al., 2011). Here, MYD88 is linked to obesity due to melanocortin 4 receptor deficiency.