The chromosome gene for mouse Mel-18 (mMel-18) protein has been isolated and characterized. The entire mMel-18 gene is composed of thirteen exons spanning about 15 kilobases, in which the protein is encoded by exons 5-13. The "RING-finger" motif of Mel-18 protein that displays a significant evolutionary resemblance to other RING-finger nuclear proteins is encoded by exons 5 and 6. Exon 13 encodes a C-terminal proline/serine-rich domain that is homologous to some transactivator proteins. Sequence analysis of the 5'-flanking region revealed the presence of potential binding sites for transcription factors such as SP-1, NF-1, NF-kappa B and c-myc/max. At least two major cap sites and three minor cap sites were identified by S1 mapping and primer extension analysis. We propose that the mMel-18 gene is regulated by two different types of promoters, the CAAT-TATA box promoter and the GC-rich TATA-less promoter. The 2.4 kb DNA fragment of the 5'-flanking region exhibited constitutive promoter activity when transfected into L cells. By the in situ hybridization method, the mMel-18 gene was assigned to mouse chromosome 10C3.