Clinical interest in the genes on human chromosome 21, especially with respect to Down syndrome (DS), has provided a strong impetus for the creation of a transcript map of this chromosome. In an effort to identify new human genes on the basis of cDNA analysis, we found several cDNA clones that corresponded to chromosome 21-specific transcripts. One of these, ha2303, showed strong similarity to the murine transcription factor Bach1. We subsequently determined the entire nucleotide sequence of this cDNA clone and found it to contain the whole coding sequence. The gene, termed BACH1, encodes a 736-amino-acid polypeptide with 80.3% identity to the murine Bach1 protein and contains a Cap'n'collar (CNC)-type basic leucine zipper (bZip) domain and a protein interaction motif, the BTB domain. Northern blot analysis revealed that BACH1 was expressed in all tissues examined. Mapping using the NotI restriction map and the YAC contig map showed that the BACH1 gene is located at 21q22.1 between the NotI sites LA329 (D21S338) and LL60 (D21S389) and within approximately 400 kb of LA329. Both the prospective function and the chromosomal location suggest that this gene may be a DS candidate gene, contributing to certain DS phenotypes, and is possibly involved in certain features of monosomy 21.