Hematopoietic stem cell enhancer: a powerful tool in stem cell biology.

There has been considerable interest in identifying a cis-regulatory element that targets gene expression to stem cells. Such an element, termed stem cell enhancer, holds the promise of providing important insights into the transcriptional programs responsible for inherent stem cell-specific properties such as self-renewal capacity. The element also serves as a molecular handle for stem cell-specific marking, transgenesis and gene targeting, thereby becoming invaluable to stem cell research. A series of candidate enhancers have been identified for hematopoietic stem cells (HSCs). This review summarizes currently known HSC enhancers with emphasis on an intronic enhancer in the Runx1 gene which is essential for the generation and maintenance of HSCs. The element, named eR1 (+24m), is active specifically in HSCs, but not in progenitors, and is hence the most definitive HSC enhancer.