Living cells respond to their environment, communicate with one another, and develop into multicellular organisms. Our lab is interested in how these tasks are accomplished using the network of interacting genes and proteins contained in the cell. We are equally interested in the opposite question of how novel networks can be engineered within cells to implement alternative cellular behaviors. We address these complementary questions together using a combination of experimental and theoretical techniques. One example of this approach is the Repressilator, a synthetic oscillatory network constructed in the bacteria Escherichia coli (Elowitz & Leibler, 2000). The Repressilator is designed to cause oscillations in the level of gene expression over time in individual cells. It consists of a negative feedback loop of three transcriptional repressors. When combined with a green fluorescent reporter gene, the Repressilator causes growing E. coli cells to flash periodically, or twinkle, demonstrating that oscillations can be genetically programmed. Interestingly, these programmed oscillations are far less regular than those of natural cellular clocks, such as the circadian clock that operates in many organisms. We are interested in how natural biological clocks behave so reliably, and conversely, in understanding what, if anything, limits the accuracy of synthetic genetic clocks.