FIGURE 2 (a) In SDS-PAGE, the cross-linked gel is contained in a device to which an electric current can be applied, such that the proteins migrate through the gel matrix. (b) A Coomassie Blue–stained SDS-PAGE gel, tracking the gradual purification of glycine N-methyltransferase. The “induced” cell extract sample is from cells that were induced to produce the protein, while the “uninduced” sample is from cells prior to inducing the protein. In practice, the protein bands are stained blue, but in publications the image is almost always shown in black and white.