Phase diagram of [protein] versus [precipitant] for protein crystallisation. The red curve is known as the ‘Solubility Curve’ and separates areas of the diagram for which the protein is either undersaturated or supersaturated. For crystallisation to occur, a given protein solution must move into the ‘Nucleation Zone’ of the supersaturated area without irreversibly entering the ‘Precipitation Zone’ (A). Upon nucleation, crystal growth is initiated and will continue as long as the protein remains in the ‘Metastable Zone’ (B). However, as the crystal grows, the protein concentration will drop until the protein remaining in solution becomes undersaturated. Note that the path taken by the protein within this two-dimensional space varies with the crystallisation technique and experimental conditions used.