In 2000 Lukyanov reported the creation of a DsRed mutant that changed from green to red over a period of 18 hours. The color change was unaffected by pH, ionic strength or protein concentration. The main drawback of this fluorescent timer (FT) was the fact that it is a tetramer. Vladislav Verkhusha took mCherry and very cleverly mutated it to obtain 3 new monomeric fluorescent timers. They change from blue to red with slow, medium and fast rates. However even the fast-FT can not be used to monitor cellular processes that occur that occur on timescales of minutes rather than hours.
To show how the fluorescent timers could be used the Verkhusha group tagged a protein called LAMP-2A, which scavenges cellular debris, with FTs and recorded how it is transported to intracellular organelles called lysosomes, where the debris are digested.
In this image, the clock numbers are colonies of bacteria that have been expressing the fast fluorescent timer protein for the number of hours indicated by the clock numeral, yielding a real-time display of the conversion from blue to red fluorescence.
(Nature Chemical Biology cover art by Erin Boyle, based on an image provided by Fedor Subach.)