From 2007.igem.org

Details of the Timed Stem Cell Self-Destruction Circuit are available on our home wiki: This circuit is designed to cause stem cells that have not differentiated and integrated into the target tissue to self-destruct.

[http://www.regensci.org/cgi-bin/twiki/view/IGEM2007/ApoptosisFinal Differentiation Circuit]

Here is an image of part of this page describing an early version of one of the two circuits for this purpose: