This work will be reported in the April 20, 2010 of PNAS . The authors have identified a compartment f extremely rare pluripotent stem cells, called Muse cells, that are capable of differentiating into muscle, nerve, and liver cells but, unlike embryonic or induced stem cells, demonstrate limited proliferation capability. After being implanted into damaged mouse muscle, nerve, or liver tissue, the Muse cells transformed into the appropriate tissue where they were transplanted--but stopped multiplying after about two weeks. Another apparent difference from other types of stem cells is an very limited capability of becoming malignant. Interesting stuff!