Used primarily in immunohistochemistry and western blotting as a chromogen substrate for horseradish peroxidase (HRP). It produces a brown precipitate at the site of enzyme activity, allowing visualization of specific proteins or antigens in tissue sections or on membranes. Commonly applied in medical and biological research for detecting low-abundance biomolecules due to its high sensitivity. Also used in electron microscopy for cellular localization studies because the reaction product is electron-dense and suitable for imaging under transmission electron microscopes. Requires careful handling as it is a suspected carcinogen.