To investigate the regulation of tumor sup-pressor XAF 1 gene expression in digestive system cancers,we studied XAF 1 gene promoter transcription activity and mRNA level in digestive system cancer cell lines (human hepatoma cell line HepG2, human colon cancer cell line LoV6, and human gastric cancer cell line AGS) and nontumor cell lines (human embryonic liver cell line L02(L02 cells) and human embryonic kidney 293 cells [HEK293 cells]) as controls. 1395-bp-promoter fragment of XAF 1 gene was amplified by polymerase chain reaction (PCR) and cloned into pGL3-basic vector and pEGFP-1vector to assay its promoter transcription activity. The plasmids were transfected into a variety of cell lines by lipofectamine 2000. The promoter transcription activity was determined by dual-luciferase report assay, and enhanced green fluorescent protein (EGFP)-positive cells were detected by fluorescence microscope. The expression of XAF1 mRNA in HEK293 and L02 were significantly higher than that in any of the three digestive system cancer cell lines. The dual-luciferase reporter assay showed that the promoter transcription activity in digestive system tumor cell lines transfected with pGL3-XAFlp promoter was apparently lower than that of both HEK293 and L02cells. Expression of green fluorescent protein (GFP) under the control of XAF 1 promoter in the three digestive system cancer cell lines was lower than that of both HEK293 and L02 cells. The activities of pGL3-XAFlp in the three digestive system cancer cell lines after treatment with heat stress were significantly lower than those in the unstressed cells. The results suggested that remarkably down-regulated XAF1 RNA expression in digestive system cancer cell lines may be due to loss of transcription activity of XAF 1 promoter.