PIN1 and hepatocellular carcinoma: Although 25 μM ATRA had moderate effects on Pin1 expression probably due to cytochrome P450-dependent ATRA metabolism in HCC cells, as we have shown [42, 43], ATRA significantly enhanced sorafenib-induced down-regulation of Pin1 expression (Figure 4A and 4B), demonstrating a synergistic effect of ATRA and sorafenib on reducing Pin1 expression.