The simple term "stem cell" actually refers to many different types of cells. The tissue-specific stem cells, or adult stem cells, replenish tissues throughout our lives. Embryonic stem cells exist only briefly in early development before tissues begin to form. Irv Weissman discovered the first tissue stem cells and has helped develop new treatments for disease based on these cells. Weissman has CIRM grants to push embryonic stem cells to form blood and heart muscle stem cells, which could be powerful tools for treating diseases of the heart or blood system (here's a list of all Weissman's CIRM awards). Weissman is Director of the Stem Cell Biology and Regenerative Medicine Institute, Virginia & D.K. Ludwig Professor for Clinical Investigation in Cancer Research, and Professor of Developmental Biology at the Stanford University School of Medicine.