What are the most commonly used chromogen in immunohistochemistry?

The most commonly used chromogens in IHC include: diaminobenzidine (DAB), Fast Red, and 3-Amino-9-ethylcarbazole (AEC). DAB is a widely used chromogen due to its ability to produce a brown precipitate upon reaction with enzymes such as horseradish peroxidase (HRP). It offers high color intensity, is very stable and is not soluble in water or alcohol. Moreover, DAB staining is heat-resistant, enabling its use in double labeling IHC/ISH experiments. It is also the most commonly used chromogen in IHC. AEC produces a red precipitate upon reaction with enzymes like HRP. This reaction occurs in a 0.05 M acetate buffer with a pH of 5 and can be easily seen with the naked eye. Fast Red is another chromogen that produces a red precipitate. It is commonly used with alkaline phosphatase (AP) enzyme systems. It can be observed using brightfield or fluorescent microscopy.