Flourescent Protein Timers

Since Reed doesn't have a subscription to Nature Chemical Biology (I guess I could Interlibrary loan it if I'm really curious--but I still find that incredibly annoying), I'm stuck reading other blogger's accounts of the cool research in there. Like florescent proteins that work as a timer. As far as I can tell, they change color temporally, making them potentially a really powerful tool for cellular tagging and localization studies.