The hematopoietic system is comprised of many different cell types that fulfill important physiological functions throughout embryonic and adult stages of mouse development. As the mature blood cells have a limited life-span, the pool of blood cells needs constant replenishing. At the basis of the hematopoietic system reside the hematopoietic stem cells (HSCs). HSCs are characterized by the ability to self-renew and to produce all the different mature hematopoietic cell types. These HSCs differentiate to the common lymphoid and myeloid progenitors (CLP and CMP) which in tum give rise to more lineage restricted precursor cells that then differentiate into mature hematopoietic cells (Figure 1 p.8). Mature hematopoietic cells can be subdivided into the lymphoid compartment, comprising T and B lymphocytes which form the adaptive immune (defense) system, and the myeloid compartment which is comprised of the oxygen transporting erythrocytes (red blood cells), blood clot-mediating platelets, immune response-mediating neutrophils, eosinophils and basophils and the macrophages, of which the main function is to remove dead and foreign cells. Although the mature blood cells fulfill the physiological functions of the hematopoietic system, the hematopoietic stem cells are essential within the adult hematopoietic hierarchy since these cells are responsible for the life long production of the different mature hematopoietic cells.