ZNF165 is a zinc-finger protein gene that was identified from human adult testis. Analysis of the origins of publicly available expressed sequence tags as presented in Unigene and SAGE databases revealed that ZNF165 mRNA was expressed in tumours of different tissues. RT-PCR, real-time PCR and Northern blotting analysis confirmed that ZNF165 mRNA was expressed in the hepatocellular carcinoma, gastric cancer, colon cancer and non-small-cell lung carcinoma. The nucleotide sequence of ZNF165 expressed in tumours is identical to that expressed in the testis. Humoral responses of hepatocellular carcinoma (HCC) patients against ZNF165 protein were determined by Western blotting using the recombinant ZNF165 protein. Antibodies against ZNF165 protein were detected in approximately 5% (four of 82) of the sera from HCC patients. These results suggest that ZNF165, a member of the ZNF family, is a novel CT antigen capable of eliciting humoral immune response and be involved in tumour biology.