The western blotting analysis showed that the phosphorylation level of MEK1/2, ERK1/2, and the protein level of a series MEK/ERK pathway proteins, including c-myc, c-fos, c-jun, CDK4, and CCND1, were upregulated (Fig. 4e), implying that Syncytin-1 upregulated the expression of MEK/ERK pathway proteins in HCC. The gene discussed is CDK4; the disease is hepatocellular carcinoma.