CD34 is a cell surface antigen of unknown function expressed in humans in hematopoietic stem cells, vascular endothelium, and blasts from 30% of patients with acute myeloid and lymphocytic leukemia. To begin to investigate the cis-acting elements required for this tissue-specific expression, the human CD34 locus was isolated and its genomic structure and transcriptional start site were characterized. The human CD34 gene spans 26 kb and has 8 exons, a structure quite similar to that of the murine gene. The start site of CD34 transcription was determined to be 258 bp upstream of the translational start site using RNase protection. These experiments also indicated that the 5' untranslated region has extensive secondary structure. In addition, fluorescence in situ hybridization was used to map the CD34 locus to band 1q32.