To search for genes related to hepatocarcinogenesis, the differential display technique for eukaryotic mRNA was conducted. We have cloned a gene that encodes the CD24 protein from the cDNA library of human hepatocellular carcinoma (HCC). A single 2.1-kb mRNA was identified in HCC specimens and the HuH-7 HCC cell line but only rarely in small amounts in nontumor livers. In 79 unicentric HCC, CD24 mRNA was overexpressed in 52 cases (66%), found in trace amounts in 11, and not detectable in 16 (20%). In 12 cases of multicentric HCC, CD24 mRNA was overexpressed in 21 (68%) of 31 tumor nodules and was helpful for the determination of tumor clonal origin. There was an increased frequency of CD24 mRNA overexpression in patients younger than 50 years with HCC (86% versus 59%, P < 0.025), in serum hepatitis B surface antigen-positive individuals (74% versus 48%, P < 0.023), in those with an elevated serum alpha-fetoprotein level (82%, versus 56%, P < 0.04), and in HCC with alpha-fetoprotein mRNA expression (82% versus 56%, P < 0.04). There was a strong correlation of CD24 mRNA overexpression with p53 gene mutation in HCC (91% versus 46%, P < 0.0005) and poorly differentiated HCC (82% versus 53%, P < 0.0008). Despite its correlation with p53 mutation and the unfavorable outcome of HCC with p53 mutation, the CD24 mRNA expression did not correlate with tumor size, tumor invasiveness, or patient's prognosis. Thus, the CD24 gene expression appears to be a common event in HCC and may serve as an early but not prognostic biomarker for malignant transformation of hepatocytes.