One key to fighting diseases such as leukemia and anemia is gaining an understanding of the genes and molecules that control the function of hematopoietic -- or blood -- stem cells, which provide the body with a constant supply of red and white blood cells and platelets. Biologists have taken a large step toward that end, with the discovery of a novel group of molecules that are found in high concentrations within hematopoietic stem cells and appear to regulate their production.