Chaperones
Chaperones assist in protein folding. Proteins' need to fold correctly is a big restriction on the evolution of protein sequences. It has been proposed that the presence of chaperones may, by providing additional robustness to errors in folding, allow the exploration of a larger set of genotypes. When chaperones are overworked at times of environmental stress, this may "switch on" previously cryptic genetic variation.
Read more about this topic: Evolutionary Capacitance