Western blots identify proteins using antibodies specific to proteins that have been first separated from one another according to their size by gel electrophoresis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is commonly used to separate proteins for use in the Western blot. The proteins are separated by weight and electrical properties as they move through a gel matrix. The Western blot test is performed after the gel-electrophoresis. The separated proteins are transferred (or blotted) onto nitrocellulose or nylon membranes and identified by specific antibodies that are tagged by a secondary protein. The specificity of the antibody-antigen interaction enables a target protein to be identified in the midst of a complex protein mixture. Western blotting can produce qualitative and semi-quantitative data about that protein.