Amplification and overexpression of genes involved in cellular growth control occur frequently in human tumors. Using a chromosome microdissection-based hybrid-selection strategy, we recently identified two novel genes (OS-9 and OS-4) within 12q13-15, a region frequently amplified in human cancers. We now report further characterization of the full-length OS-9 cDNA sequence consists of 2785 bp from which an open reading frame (ORF) with 667 amino-acid residues was deduced, The predicted polypeptide was water soluble and acidic. We also demonstrate that the OS-9 gene encoded a 2.8-kb mRNA transcribed in all 16 human tissues examined, suggesting that OS-9 is ubiquitously expressed in human tissues. OS-9 was co-amplified with CDK4 in three of five sarcoma tissues. Homology analysis of the amino-acid sequence reveals significant similarities between OS-9 and two ORFs deduced from genomic sequences in Caenorhabditis elegans and Saccharomyces cerevisiae. The region of similarity extended over 200 residues (approximately one-third of each ORF), and eight cysteines were conserved in all three ORFs. These observations suggest that this region comprises a functional domain present in a novel evolutionarily conserved gene family defined by OS-9.