The transcription factor PU.1 (also known as Spi-1) plays a critical role in the development of the myeloid lineages, and myeloid cells derived from PU.1(-/-) animals are blocked at the earliest stage of myeloid differentiation. Expression of the PU.1 gene is tightly regulated during normal hematopoietic development, and dysregulation of PU.1 expression can lead to erythroleukemia. However, relatively little is known about how the PU.1 gene is regulated in vivo. Here it is shown that myeloid cell type-specific expression of PU.1 in stable cell lines and transgenic animals is conferred by a 91-kilobase (kb) murine genomic DNA fragment that consists of the entire PU.1 gene (20 kb) plus approximately 35 kb of upstream and downstream sequences, respectively. To further map the important transcriptional regulatory elements, deoxyribonuclease I hypersensitive site mapping studies revealed at least 3 clusters in the PU.1 gene. A 3.5-kb fragment containing one of these deoxyribonuclease I hypersensitive sites, located -14 kb 5' of the transcriptional start site, conferred myeloid cell type-specific expression in stably transfected cell lines, suggesting that within this region is an element important for myeloid specific expression of PU.1. Further analysis of this myeloid-specific regulatory element will provide insight into the regulation of this key transcriptional regulator and may be useful as a tool for targeting expression to the myeloid lineage.