Since numerous lipid and fatty acid metabolic enzymes/proteins regulate cell steatosis [21], we analyzed the effect of serotonin on the expression of lipid/fatty acid metabolic proteins, including fatty acid synthase (FAS), liver fatty acid-binding protein-1 (L-FABP1), acetyl-CoA carboxylase, (Acc), stearoyl-CoA desaturase-1 (SCD1), and transcription factors such as sterol regulatory element-binding protein 1 (SREBP1), peroxisome proliferator-activated receptor alpha and gamma (PPARα and PPARγ). The gene discussed is FAS; the disease is steatosis.