AIM: To search for the biomarker of cellular immortalization,the telomere length, telomerase activity and its subunitsin cultured epithelial cells of human fetal esophagus in theprocess of immortalization.METHODS: The transgenic cell line of human fetalesophageal epithelium (SHEE) was established with E6E7genes of human papillomavirus (HPV) type 18 in ourlaboratory. Morphological phenotype of cultured SHEEcells from the 6th to 30th passages, was examined byphase contrast microscopy, the telomere length wasassayed by Southern blot method, and the activity oftelomerase was analyzed by telomeric repeatamplification protocol (TRAP). Expressions of subunitsof telomerase, hTR and hTERT, were assessed by RTPCR.DNA content in cell cycle was detected by flowcytometry. The cell apoptosis was examined by electronmicroscopy (EM) and TUNEL label.RESULTS: SHEE cells from the 6th to 10th passages showedcellular proliferation with a good differentiation. Fromthe 12th to the 16th passages, many senescent andapoptotic cells appeared, and the telomere lengthsharply shortened from 23kb to 17kb withoutexpression of hTERT and telomerase activity. At the20th passage, SHEE cells overcame the senescence andapoptosis and restored their proliferative activity withexpression of telomerase and hTERT at low levels, butthe telomere length shortened continuously to thelowest of 3kb. After the 30th passage cells proliferationwas restored by increment of cells at S and G2M phasein the cell cycle and telomerase activity expressed athigh levels and with maintenance of telomere length.CONCLUSION: At the early stage of SHEE cells, telomeresare shortened without expression of telomerase and hTERTcausing cellular senescence and cell death. From the 20thto the 30th passages, the activation of telomerase andmaintenance of telomere length show a progressive processfor immortalization of esophageal epithelial cells. Theexpression of telomerase may constitute a biomarker fordetection of immortalization of cells.