Using high-density oligonucleotide microarrays, we investigated DNA copy-number aberrations in cell lines derived from hepatocellular carcinomas (HCCs) and detected a novel amplification at 17p11. To identify the target of amplification at 17p11, we defined the extent of the amplicon and examined HCC cell lines for expression of all seven genes in the 750-kb commonly amplified region. Mitogen-activated protein kinase (MAPK) 7, which encodes extracellular-regulated protein kinase (ERK) 5, was overexpressed in cell lines in which the gene was amplified. An increase in MAPK7 copy number was detected in 35 of 66 primary HCC tumors. Downregulation of MAPK7 by small interfering RNA suppressed the growth of SNU449 cells, the HCC cell line with the greatest amplification and overexpression of MAPK7. ERK5, phosphorylated during the G2/M phases of the cell cycle, regulated entry into mitosis in SNU449 cells. In conclusion, our results suggest that MAPK7 is likely the target of 17p11 amplification and that the ERK5 protein product of MAPK7 promotes the growth of HCC cells by regulating mitotic entry.