(1) A metabolite of lactose binds to the lac repressor, changing the protein's shape and thereby causing it to loosen its grip on the DNA.(2) These strains might not even use their lac gene products to metabolize lactose.(3) This system is probably still the best-understood regulatory system in eukaryotic biology and today is taught in every textbook of genetics and cell biology, alongside the lysis-lysogeny decision of and the lac operon of E. coli.(4) In addition to the lac operon, a number of other synthetic and natural genetic circuits have been analyzed for robustness and evolvability.(5) Other events are correlated with the stable exploration of new niches, like the acquisition of the lac operon by E. coli or pathogenicity islands by Salmonella.