How do scientists know when they have found a stem cell? Every cell displays an array of proteins on its surface; different cell types have different proteins. Scientists can use these surface proteins as "markers" that characterize individual cell types-a type of "molecular ID". For example, using molecules that recognize and attach to specific surface proteins and that can fluoresce under certain wavelengths of light, scientists can visually tell the difference between a blood stem cell and a mature white blood cell. Unfortunately, not all stem cells can now be identified in this manner because scientists have not yet identified markers for all stem cell types. Scientists also identify stem cells by observing their behavior in the laboratory: stem cells must be able to remain unspecialized and self-renew for long periods of time. Scientists believe that there might be more types of adult stem cells than the handful that have already been identified, but finding them is a difficult process.
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