To disclose mechanisms of hepatocellular carcinogenesis and to identify novel diagnostic markers and/or drug targets for treatment of hepatocellular carcinomas (HCCs), we analyzed expression profiles of clinical HCCs using a genome-wide cDNA microarray. From among the transcripts that were commonly up-regulated in these tumors we identified a novel human gene at chromosomal band 1p36.13, termed DDEFL1 (development and differentiation enhancing factor-like 1), encoding a product that shared structural features with centaurin-family proteins. The deduced 903-amino acid sequence showed 46% homology to DDEF/ASAP1 (development and differentiation enhancing factor), and contained an Arf GTPase-activating protein (ArfGAP) domain and two ankyrin repeats. Gene transfer of DDEFL1 promoted proliferation of cells that lacked endogenous expression of this gene. Furthermore, reduction of DDEFL1 expression by transfection of anti-sense S-oligonucleotides inhibited the growth of SNU475 cancer cells, in which DDEFL1 expression was highly up-regulated. Our results provide novel insight into hepatocarcinogenesis and may contribute to development of new strategies for diagnosis and treatment of HCC.