Irving L. Weissman, MD, has directed the Institute for Stem Cell Biology since its founding, providing vision and leadership to build one of the nation’s top stem cell programs. In 1988, Dr. Weissman became the first to isolate in pure form any stem cell in any species when he isolated the hematopoietic or blood-forming stem cell in mice. He subsequently isolated the human hematopoietic stem cell, the human neuronal stem cell, and the human leukemia stem cell. His work has opened up an entirely new area of scientific research with enormous potential for life-saving therapies. Dr. Weissman recently made an exciting step toward the goal of transplanting adult stem cells to create a new immune system for people with autoimmune or genetic blood diseases. As published in the November 2007 issue of Science, his lab found a novel way to transplant new blood-forming stem cells into the bone marrow of mice without the tissue-damaging radiation or chemotherapy usually required, thereby effectively replacing their immune systems. Many aspects of this technique will need to be adapted before it can be tested in humans, but when those barriers are surmounted, the benefits could be significant. An immune system transplant, much like a liver or heart transplant, would give a person with an autoimmune disease, such as multiple sclerosis, hope for a healthy future. Dr. Weissman received his medical degree from Stanford in 1965 and, after carrying out research in laboratories provided by the late Henry S. Kaplan, MD, joined the faculty four years later. In addition to being the Virginia and D. K. Ludwig Professor for Clinical Investigation in Cancer Research, he is a professor of pathology and developmental biology, and, by courtesy, professor of neurosurgery and of biological sciences.