Two-dimensional gel electrophoresis of a complex mixture of proteins. In the first dimension, proteins are separated on a gel strip according to charge (i.e., pI) by isoelectric focusing. The strip is then laid on top of an SDS-polyacrylamide gel, and the proteins are separated according to size (i.e., molecular mass) by electrophoresis. The original protein complement is thus spread in two dimensions, aiding the separation of similar proteins into individual spots. The spots can be cut out of the gel and the proteins identified by mass spectrometry.