3,3′,5,5′-Tetramethylbenzidine (TMB) dihydrochloride hydrate is a chromogenic substrate commonly used in enzyme-linked immunosorbent assays (ELISAs) for detecting the presence of horseradish peroxidase (HRP). When TMB is oxidized by HRP in the presence of hydrogen peroxide, it produces a blue color, which can be quantitatively measured spectrophotometrically. This color change is used to detect the presence of HRP-labeled antibodies or other proteins in ELISA experiments. TMB is widely used in research, diagnostics, and pharmaceutical industries for its sensitivity and ease of use in detecting various biomolecules.