Milestones in directed enzyme evolution.

Directed evolution has now been used for over two decades as an alternative to rational design for protein engineering. Protein function, however, is complex, and modifying enzyme activity is a tall order. We can now improve existing enzyme activity, change enzyme selectivity and evolve function de novo using directed evolution. Although directed evolution is now used routinely to improve existing enzyme activity, there are still only a handful of examples where substrate selectivity has been modified sufficiently for practical application, and the de novo evolution of function largely eludes us.