Two-dimensional electrophoresis (2-DE) of proteins is used for several purposes, such as resolving a distinct group of proteins (e.g., serum proteins), revealing the heterogeneity of a particular protein (e.g., actin, transferrin), purifying a protein, or testing the purity of a protein gained by other methods. However, the most exceptional feature of this method is its potentiality to resolve all the various proteins of a certain cell type or tissue. It was this particular feature of the 2-DE technique developed in 1975 (1,2) that opened the way to study many of the biological problems of present interest under a new aspect: Problems of gene expression, gene regulation, genetic variation, cell differentiation, embryonic development, pathogenesis of certain diseases, and other fields could be studied on the basis of a broad spectrum and a representative number of proteins, whereas previously studies like these were performed on selected proteins, often selected because of their easy accessibility and considered as model proteins (e.g. hemoglobin). Moreover, when simple procedures for extracting cell or tissue proteins were used, i.e., procedures that avoid steps, such as protein precipitation, lyophilization, dialyses, or chromatographic fractionations, 2-DE protein patterns revealed the individual proteins in quantities that reflect, at least to some extent, the relative concentration of these proteins in the cells or tissues. Quantitative data on gene expression offer an important parameter for all studies in cell biology.