Comparing different proteomes by classical two-dimensional electrophoresis is challenging and often complicated by substantial gel-to-gel variation. Separating two or more protein samples labelled with different fluorescent dyes in one single gel, as in two-dimensional difference gel electrophoresis, reduces this variability considerably. Recent technological innovations, specifically the introduction of a pooled internal standard, even further improve the quantification accuracy and statistical confidence of this method. In addition, decreasing the sample complexity by one of several protein or organelle fractionation procedures increases the number of spots investigated by this protein differential display methodology.