DEC1/BHLHB2 is a novel cAMP-inducible basic helix-loop-helix (bHLH) transcriptional factor isolated from human chondrocyte cultures by the subtraction method [Shen et al. (1997) Biochem. Biophys. Res. Commun. 236, 294--298]. DEC1 seems to be involved in controlling the proliferation/differentiation of some cell lineages. We determined the structure of the human DEC1 gene and its chromosomal locus. Phylogenetic analysis and comparison of the gene structure showed that the DEC1 protein is a member of a new subgroup of the proline bHLH protein family that diverged earlier than other proline bHLH proteins including HES, hairy and E(spl). The human DEC1 gene spans approximately 5.7 kb and contains 5 exons. The putative promoter region contains multiple GC boxes but no TATA box. A primer extension study showed multiple transcriptional initiation sites. In the 5'-flanking region of the DEC1 gene, several transcriptional factor binding sites, including a cAMP-responsive element (CRE), were found using the transcription factor database. The DEC1 gene locates at Chromosome 3p25.3--26 by the FISH method. This is the first study to determine the genomic structure of the DEC1 gene subgroup.