Certain cells may have endogenous enzymes that can react with the chromogens used in the staining procedure. This reaction can produce a false-positive result. This is common with horseradish peroxidase (HRP) and mainly alkaline phosphatase (AP) applications. Tissues such as intestinal, placental, and kidney, to name a few, have high amounts of endogenous AP activity. An enzyme block is necessary when using an AP method. Also, frozen sections usually have high endogenous AP activity. To quench endogenous AP activity, a 5 mM levamisole in the chromogen substrate solution may be used.