Rational design of memory in eukaryotic cells
View pdf - Rational design of memory in eukaryotic cells Caroline M. Ajo-Franklin, et al. 2007.
Keep in mind that the first half of the paper discusses the creation of an activator. This construct is intended to produce both red and yellow fluorescence in the presence of galactose. The second part of the paper discusses how this activator is then modified to serve as a sythetic memory device with three separate states:
Yeast that have never been in the presence of galactose will not fluoresce.
Yeast that are currently in the presence of galactose will fluoresce red (and yellow).
Yeast that have at one time been in the presence of galactose will fluoresce yellow.
Note, again, that in this paper, the creation of a bistable system relies on cooperative binding  (in this case, cooperative binding of an activator). Pay attention to this when looking over the math modeling.