DP-1 is a heterodimerization partner for members of the E2F family of transcription factors; E2F/DP-1 regulates the expression of various cellular promoters, particularly gene products that are involved in the cell cycle. Our earlier studies identified the DP-1 gene ( TFDP1) as a probable target within a 13q34 amplicon that is frequently detected in hepatocellular carcinomas (HCC) and esophageal squamous-cell carcinomas. The aim of the present study was to investigate the clinicopathological significance of up-regulation of TFDP1 in HCC. We determined expression levels of TFDP1 and E2F1 in 41 primary HCCs by means of quantitative real-time reverse transcription-polymerase chain reactions, and looked for relationships between those data and various clinicopathological parameters. To assess transactivating activity of E2F1/DP-1, we also analyzed expression of ten putative transcriptional targets of this complex in HCCs. Using antisense oligonucleotides, we down-regulated TFDP1 in Hep3B, an HCC cell line that had shown overexpression of the gene, to examine the role of elevated TFDP1 expression in the growth of Hep3B cells. Elevated expression of TFDP1, but not E2F1, was associated significantly with large (> or =5 cm) tumor size (P=0.021). Expression levels of TFDP1 and E2F1 correlated with those of seven transcriptional targets ( TYMS, DHFR, PCNA, RRM1, CCNE1, CDC2, and MYBL2) that play important roles in the G1/S transition, and down-regulation of TFDP1 inhibited growth of Hep3B cells. In conclusion, overexpression of TFDP1 may contribute to progression of some HCCs by promoting growth of the tumor cells.