Serological analysis of cDNA expression library (SEREX) was employed to identify cancer-associated genes. By screening cDNA expression libraries with sera of patients with lung cancers, we identified a total of 49 genes that specifically reacted with the sera of patients with lung cancers. Among these, we characterized a novel gene with expression pattern similar to that of cancer/testis antigens. Its open reading frame is 1920 bp in size and encodes for putative protein composed of 639 amino acids. Southern blot analysis reveals that this gene exists as single copy. In vitro transcription/translation and Western blot analysis confirm that this gene encodes a protein of 73 kDa in size. The comparison of cDNA and genomic sequences reveals that it is composed of 11 exons and 10 introns. This gene displays testis-specific expression among normal tissues, and wide expression among various cancer tissues and cancer cell lines. A study using GFP fusion construct reveals mainly nuclear localization of CAGE-1 protein. The expression of this clone is relatively higher in cancer tissues compared with their surrounding non-cancerous tissues. This suggests that overexpression of CAGE-1 may be associated with the progression of tumor. Because of its association with cancer, this gene was named cancer-associated gene-1 (CAGE-1). Given the fact that several cancer/testis antigens reportedly induce cytolytic T lymphocyte (CTL) reactions, it is reasonable that this gene will be a valuable target for cancer immunotherapy. The exact functional role of CAGE-1 in tumorigenesis remains to be seen.