Perhaps a picture is worth a thousand back Science words. Venn diagrams in back-to-papers from 2002 feature three intersecting circles representing gene-expression profiles shared among murine hematopoietic stem cells (HSCs), neural stem cells (NSCs), and embryonic stem cells (ESCs). These profiles define genes that the researchers term a molecular signature, or parts list, for "stemness." Since then, many studies have extended their results. "The global gene-profiling approach to identifying and defining stem cells is alive and well," says James Sherley, associate professor in the biological engineering division of the Massachusetts Institute of Technology.