In order to contribute to the development of the transcriptional map of chromosome 21, we performed exon trapping using cosmid clones mapped in the region 21q22.1-22.2 and identified a number of potential exons. One of the trapped exons (Genbank No. AF026200) showed a strong homology with the mouse Bach1 gene (Genbank No. D86603), a transcription factor regulating gene expression. We then isolated the full-length coding region of the human BACH1 gene using expressed sequence tags, reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends. The predicted BACH1 protein contains 736 amino acids and is 88% identical to its mouse homolog. It contains basic leucine zipper and BTB-zinc finger domains (which are directly involved in DNA binding for transcription regulation). The BACH1 gene maps in a relatively gene-poor region on 21q22.1 in yeast artificial chromosome 814c1 of the collection of Chumakov et al. Northern blot analysis revealed that it is expressed as an mRNA species of approximately 5.8 kb in all 16 adult and 4 fetal tissues examined; an additional mRNA species of 2.8 kb was observed in adult testis. The contribution of the BACH1 gene to the pathophysiology of trisomy or monosomy 21 is unknown. In addition, no monogenic disorders associated with mutations in the BACH1 gene have yet been identified.