To identify genes differentially expressed in association with metastatic potential of K-1735 mouse melanoma cells, the mRNA differential display method was applied to compare mRNAs from high- and low-metastatic K-1735 cells. A novel gene was identified as being expressed in high-metastatic cells but not in low-metastatic cells. Sequence analysis revealed that this gene had an open reading frame of 538 amino acid residues containing a Sry high-mobility group (HMG) box domain, known as a DNA binding motif, particularly in the Sox family genes. This gene showed the highest homology to the human SOX9 gene, which is the responsible gene for an inherited disease, campomelic dysplasia. Thus, this gene was designated the Sox21 gene (the 21st Sox family gene). The Sox21 gene was mapped to mouse Chromosome 15, to which neither genes containing the HMG box region nor the loci of hereditary diseases similar to campomelic dysplasia have been previously mapped. This gene was highly conserved and specifically expressed in the brain. These results suggest that expression of the Sox21 gene is involved in the development of nerve systems and may function to enhance the metastatic potential of K-1735 mouse melanoma cells of nerve cell origin.