This technique fractionates proteins on the basis of their binding to and elution from a hydrophobic matrix, commonly octyl- or phenyl-agarose. Binding of the proteins is often carried out at high salt concentration to favour hydrophobic interactions. Some proteins may precipitate at this high ionic strength and so need to be removed by centrifugation prior to loading the protein mixture onto the column. Selective elution of bound proteins is then carried out by applying a decreasing salt gradient. Proteins that fail to elute at low salt concentration may be eluted by washing the column with aqueous ethylene glycol, ethanol or certain chaotropic agents such as urea, but this is not simulated in this program. This technique is therefore like other general chromatographic procedures such as ion-exchange chromatography and gel filtration in that it relies on differences in a particular physical property of the protein molecules being separated.