The most commonly used enzymes as labels for ELISA assays are horseradish peroxidase and alkaline phosphatase. These enzymes are used as labels because they produce colored compounds. More specifically, horseradish peroxidase and alkaline phosphatase conjugated antibodies are typically used along with a chromogenic substrate (e.g. TMB) for the detection of antigens or proteins. The substrates are used so that they will produce a chemiluminescent, fluorescent, or chromogenic signal during oxidation. Additional enzymes used as labels for ELISA assays include acetylcholinesterase, catalase, and b-galactosidase. It is important to note that the substrate being used can vary depending on the assay sensitivity and instrument being used.