Although the critical requirement for the transcription factor RUNX1/AML1 at the onset of hematopoietic development is well established, little is known about its transcriptional targets at this pivotal stage of blood development. Using microarrays, we identified the uncharacterized gene AI467606 as a gene whose expression level is dramatically reduced in the absence of RUNX1. We further demonstrated by chromatin immunoprecipitation and promoter assay a direct regulation of its transcription by RUNX1. Using a bacterial artificial chromosome transgenic approach, we established that AI467606 is expressed during the development of the hematopoietic system in vivo and in vitro and that its expression is detected within the CD41(+) population and marks definitive hematopoietic potential. Similarly, in the adult mouse, all hematopoietic cell lineages, except mature erythrocytes, express AI467606. Taken together, these findings indicate that AI467606 is a novel transcriptional target of RUNX1/AML1 at the onset of hematopoietic development that is extensively expressed within the hematopoietic system.