For many applications proteins must retain their function for a long time and under a wide range of conditions, sometimes at elevated temperatures. However, the stability of natural proteins is often very low, just sufficient to ensure proper functioning under cellular conditions (1 ). It is still largely unknown how the stability of a protein is encoded in its sequence, and theoretical approaches to calculate the contributions of individual amino acids to stability are still in their infancy. Therefore, evolutionary methods for increasing protein stability are of great interest. In such approaches, large protein libraries are searched for stabilized variants by a screening or, ideally, a selection technique.
