Recent developments in stem cell biology have generated much excitement about the potential for cell-based therapies in a variety of clinical applications, such as treating Parkinson’s disease, leukemia, and spinal cord injuries (Orkin, 2000). Crucial to the success of these therapies is the detailed understanding of how the cells remain stem cells, and the cues that they require to commit themselves to specific cell fates.

In hematopoiesis, mature differentiated cells are continuously replaced by stem cells. These stem cells reside in the bone marrow, and have the ability to self-renew as well as to differentiate into a wide variety of blood cell types, a process that is very tightly regulated. During differentiation, cells become committed to a lineage fate, and lose the ability to differentiate into an alternate cell lineage. There are catastrophic consequences to aberrant hematopoiesis, such as leukemia, lymphoma, etc. A variety of studies using GeneChip® arrays have focused on the normal cell development or on the cell-lineage switch (Schweitzer, 2004; Araki, 2004). Understanding the nature of stem cells, as well as the molecular process by which these cells acquire their specific cell fate is crucial to the success of cell-based therapies.