HEMATOPOIETIC STEM CELLS The hope that many diseases can someday be treated with stem cell therapy is inspired by the historical success of bone marrow transplants in increasing the survival of patients with leukemia and other cancers, inherited blood disorders, and diseases of the immune system (Thomas and Blume, 1999). Nearly 40 years ago, the cell type responsible for those successes was identified as the hematopoietic stem cell (Till and McCullough, 1961). The ability of hematopoietic stem cells (HSCs) to self-renew continuously in the marrow and to differentiate into the full complement of cell types found in blood qualifies them as the premier adult stem cells (). Blood Cell Differentiation from Hematopoietic Stem Cells (HSCs). HSCs normally divide to generate either more HSCs (self-renewal) or progenitor cells, which are precursors to various blood cell types.