Horseradish Peroxidase (HRP) is an enzyme commonly used as an indicator for chemical reactions which produce peroxide. The enzyme is routinely conjugated to antibodies for use in enzyme-based immunoassay systems.
Fluorescein isothiocyanate (FITC) is a derivative fluorescein used in wide-ranging applications including flow cytometry.
Biotin binds to streptavidin and avidin with extreme high affinity, and high specificity.
Biotin is used in detection of the protein via anti-biotin antibodies or avidin/streptavidin-tagged detection strategies such as enzyme reporters (Horseradish Peroxidase (HRP), Fluorescein isothiocyanate (FITC)) or fluorescent probes (DY Quenchers).