Similar to other nuclear hormone receptores, PPAR acts as a ligand activated transcription factor. Upon binding fatty acids or hypolipidemic drugs, PPARa interacts with RXR and regulates the expression of target genes. These genes are involved in the catabolism of fatty acids. Conversely, PPARg is activated by prostaglandins, leukotrienes and anti-diabetic thiazolidinediones and affects the expression of genes involved in the storage of the fatty acids. PPARb is only weakly activated by fatty acids, prostaglandins and leukotrienes and has no known physiologically relevant ligand. However, data from PPARb null mice suggest PPARb does serve a role in fatty acid metabolism and perhaps in skin proliferation and cancer.