INTRODUCTION

Homeostasis of the hematopoietic system depends on the ordered replenishment of terminally differentiated cells. This process relies on rare hematopoietic stem cells （HSCs） and their progeny, which continuously enter tightly controlled pathways of lineage commitment, differentiation, and maturation. In steady-state hematopoiesis, HSCs are believed to be （a） relatively quiescent with respect to cell division frequency , evidence in support of the "traditional" model is provided by a large number of transplantation experiments using cell populations highly enriched for HSCs.

CONCLUSION

Gene expression profiling studies from murine hematopoietic, otherwise somatic, and embryonic stem cells have defined a putative set of stemness genes. Validation of these results by means of hematopoiesis-specific gain- and loss-of-function analyses is presumably being pursued by many groups. In this regard, the availability of a resource like the cDNA libraries described herein may further HSC research in many ways. The value of this resource is underscored by its accordance with previous HSC gene expression studies. Furthermore, careful analysis of the cDNA libraries contents suggested that steady-state HSCs are far more "active" than anticipated.