Protein crystallization for X-ray structure studies has long been an art rather than a science. In recent years it was realized that a certain effective attraction between the protein molecules must be induced. Traditionally, attractions were induced by varying ionic strength and pH (salting out). Actually, this was mainly influencing enthalpic interactions. Nowadays, polymers are added to tune both range and depth of the interaction. By choosing the size (radius of gyration) of the polymer and its concentration, optimum conditions for protein crystallization can be selected, see