Human chromosome 21 has been extensively studied because trisomy 21 causes Down syndrome. In addition to this syndrome, chromosome 21 is the loci for five other hereditary disorders, myoclonus epilepsy, autoimmune polyglandular disease type I, nonsyndromic hereditary deafness, Knobloch syndrome, and bipolar affective disorder. Exon trapping studies of human chromosome 21q22.3 identified transcriptional units with homology to the zebrafish ES1 and the E. coli sigma cross-reacting protein 27A (SCRP27A). The human ES1 homolog (HES1) was also identified as KNP-1α/KNP-1β, as well as GT335 in similar screens for genes involved in disorders that map to chromosome 21q22.3. KNP-1/HES1 mRNA is ubiquitously expressed with the highest expression in muscle and heart. KNP-1/HES1 protein contains putative mitochondrial targeting signals and localizes to mitochondria in mouse NIH 3T3 cells. Interestingly, mitochondrial deletions have been implicated in syndromes related to deafness. Thus, KNP-1/HES1 may be a mitochondrial protein important for normal development, and may be a candidate gene for disorders related to deletions at chromosome 21q22.3.