Synthetic biologists have installed a genetic feedback loop in yeast, the first of these circuits to be built in a eukaryotic cell (one with membrane-bound structures, such as a nucleus). The loop consisted of two novel genes. When exposed to the sugar galactose, the yeast cell would activate the first implanted gene; that gene in turn would create a transcription factor that switched on the second gene. The second gene would then manufacture its corresponding transcription factor, which was designed to ratchet up the activity of the same gene that created it. Because of this looping effect, the second gene kept making its transcription factor even when galactose was eliminated from the cell's environment, effectively sustaining the memory of galactose exposure. Cells with such feedback designs could be jury-rigged to record past environmental conditions or quantify DNA damage. The report appears in the September 15 Genes & Development.