One of the more intriguing features of the hemopoietic system is the ability of a single phenotypically homogeneous stem cell to give rise—through differentiation and development—to all the phenotypically diverse mature immuno-hemopoietic cell types (1 ). The stem cell therefore sits at the heart of the hemopoietic system, and it is at this cellular level that the entire complex system can ultimately be regulated. For this reason, an understanding of the nature of the factors acting to regulate processes such as stem cell proliferation is central to our overall ability to understand the functioning of the hemopoietic system (2 ).