Hematopoietic stem cells can grow and make healthy blood cells after being transplanted into patients with blood cell–related diseases. Researchers have now revealed the unique signature of genes expressed by these cells. The findings could enable scientists to expand these cells or to convert other types of cells into cells that can repopulate the blood system, which would have important clinical applications. This work could facilitate treatment of multiple diseases or allow the creation of a limitless supply of these rare cells from other more common cell types in the future.