Mouse Wnt-3 is a proto-oncogene, which is activated by mouse mammary tumor virus (MMTV). Human WNT3 cDNA fragment, previously isolated by another group, corresponds to a partial coding sequence. WNT3 cDNA, spanning the complete coding sequence, was isolated in this study. WNT3 encoded 355-amino-acid polypeptide with the N-terminal signal peptide and two N-linked glycosylation sites. WNT3 was most homologous to WNT3A (84.2% total amino-acid identity) among human WNTs. The WNT3 gene on the human chromosome 17q21 region consisted of five exons. WNT3 mRNAs were detected in fetal brain, adult brain, and testis by Northern blot analyses. WNT3 mRNA was relatively highly expressed in A549 cells (lung cancer) and MKN45 cells (gastric cancer) among 37 human cancer cell lines. WNT3 was significantly up-regulated in a case of primary breast cancer and in a case of primary rectal cancer among various types of human primary cancers. These results strongly suggest that WNT3 might play a key role in some cases of human breast, rectal, lung, and gastric cancer through activation of the WNT - β-catenin - TCF signaling pathway, similar to mouse Wnt-3.