SOX proteins are a family of transcription factors with high-mobility-group DNA-binding domain (HMG box) homologous to SRY, which play key roles in embryogenesis. Xenopus Sox17alpha, Sox17beta, Sox3 and mouse Sox7 are reported to be negative regulators of the WNT-beta-catenin-TCF signaling pathway. SOX7, SOX17, and SOX18 constitute a subfamily among the SOX gene family. Here, expression of SOX18 mRNA was investigated using Northern blot analysis, RNA dot blot analysis, and cDNA-PCR. SOX18 mRNA was significantly highly expressed in ventricles and inter-ventricular septum of adult heart among various normal human tissues. SOX18 mRNA was relatively highly expressed in stomach and jejunum in the gastrointestinal tract. SOX18 mRNA was relatively highly expressed in TMK1 and MKN45 among 7 gastric cancer cell lines. SOX18 mRNA was expressed in all out of 7 pancreatic cancer cell lines, and was relatively highly expressed in PANC-1, Hs700T, Hs766T and MIA PaCa-2. Expression level of SOX18 mRNA in MCF-7 cells (breast cancer) was not affected by beta-estradiol. SOX18 mRNA was expressed in all out of 5 embryonal tumor cell lines, and was relatively highly expressed in NT2 with the potential to differentiate into neuronal cells. Expression level of SOX18 mRNA in NT2 cells was down-regulated by all-trans retinoic acid. This is the first report on comprehensive expression analyses of SOX18 mRNA in normal human tissues and tumors.