ERROR MINIMIZATION IN BIOLOGY. “Savageau demand rule: Genes that are needed often in the natural environment tend to be regulated by activators, and rarely needed genes tend to be regulated by repressors; in both cases, sites are bound for most of the time, and errors are minimized. The fitness advantage of error minimization appears to be readily selectable.”
http://www.pnas.org/content/103/11/3999.full