To gain new insight into the molecular mechanism underlying the pathogenesis of human primary hepatocellular carcinoma (HCC), we searched for HCC-specific molecules through screening genes that are differentially expressed between cancerous and noncancerous counterparts of liver and identified a novel HCC-associated gene, HCCA1 encoding a approximately 80 kDa cytoplasmic protein that contains several proline-rich motifs likely for SH3-binding. HCCA1 transcript, albeit present in some adult tissues, is up-regulated selectively in HCC but not in other tumor cells. High expression of HCCA1 occurs as a late event frequently (89.2%) in HCCs and correlated significantly with the degree of tumor progression. When treated with antisense oligonucleotides to HCCA1, HCCA1 expression in HCC cells (HuH-7) was effectively suppressed and cell growth was down-regulated in a time- and dose-dependent manner. Furthermore, HuH-7 cells harboring the HCCA1 antisense expression clone displayed a remarkably reduced efficiency in colony formation. Together, these data strongly suggest that HCCA1 is a positive effector in cell proliferation and contributes to HCC carcinogenesis and progression. We believe that this protein will serve as a novel useful marker for HCC and is a potential target for pharmaceutical intervention of this malignant disease.