Back to the beginning--the quest for thymic epithelial stem cells.

Stem cell-based therapies hold much promise for the rejuvenation of aged or damaged tissues; however, before such cells can be used therapeutically, they must first be accurately identified. In this issue of the European Journal of Immunology it is reported that MTS24, a marker previously associated with progenitor cells of the thymic epithelium, fails to accurately identify epithelial cell populations with the ability to reconstitute a functional thymus. This finding demonstrates that much progress needs to be made before thymic epithelial stem cells can be harnessed for clinical benefit.