To identify potential oncofetal biomarkers that distinguish hepatocellular carcinoma (HCC) from healthy liver tissues, we compared and analyzed the proteomic profiles of mouse livers at different developmental stages. Fetal (E13.5, E16.5), newborn (NB), postnatal (3-week) and adult (3-month) livers were isolated and profiled by 2-D PAGE. Statistical analysis using linear regression and false discovery rate (FDR) revealed that 361 protein spots showed significant changes. Unsupervised hierarchical tree analysis segregated the proteins into fetal, NB, and postnatal-adult clusters. Distinctive protein markers were identified by MALDI-TOF/MS and the corresponding mRNA profiles were further determined by Q-PCR. Fetal markers (hPCNA, hHSP7C, hHEM6) and postnatal-adult markers (hARGI1, hASSY, hBHMT, hFABPL) were selected for testing against a panel of seven human hepatocyte/HCC cell lines and 59 clinical specimens. The fetal proteins were found to be overexpressed in the metastatic HCC cell lines and the tumor tissues, whereas the postnatal-adult proteins were expressed in non-tumor tissues and normal hepatocytes. This "Ying-Yang" pattern, as orchestrated by distinct fetal and adult markers, is hypothesized to indicate the progressive change of the liver from a growing, less-differentiated organ into a functional metabolic center. Thus, embryogenesis and tumorigenesis share certain oncofetal markers and adult "hepatic" phenotypes are lost in HCC.