Western blotting. Proteins on an SDS–polyacrylamide gel are transferred to a polymer sheet. The sheet is first treated with a primary antibody, which is specific for the protein of interest, and then washed to remove unbound antibody. Next, the sheet is treated with a secondary antibody, which recognizes the primary antibody, and washed again. Since the secondary antibody is labeled (here, with a fluorescent tag indicated by the yellow circle), the band containing the protein of interest can be identified.