Gel electrophoresis is the process of separating proteins and small nucleic acids by charge and size. Precast Polyacrylamide electrophoresis gels are available in various densities and gradient densities. Precast Agarose gels are also available. Proteins can have various charges depending upon the surface of the folded protein. A surfactant such as sodium dodecyl sulfate (SDS) is added to the sample and the gels to denature the protein and give it a negative charge. This will ensure that the proteins are separated on the basis of size only. A gel electrophoresis system consists of a buffer tank, a cassette with electrical leads for supplying charge to the gel, and a power supply. Electrophoresis can be vertical or horizontal. The gels must be stained to visualize the proteins. Silver nitrate or a dye such as Coomassie Blue are used to help visualize the proteins in the gel. Each gel is run with a molecular weight standard so that the molecular weights can be estimated for the proteins of interest.