HRP corresponds to the peroxidase encoded by the Armoracia rusticana (horseradish) HRP gene. It can be used as a genetically encoded tag for electron microscopy (EM) or as a proximity labeling tool for proteomic analysis. HRP is not active in the cytosol or other reducing cellular environments, limiting its usage to extracellular or secretory pathway compartments. For EM, cells expressing HRP are fixed and then exposed to diaminobenzidine (DAB) and hydrogen peroxide. HRP catalyzes the hydrogen peroxide-dependent polymerization and local deposition of DAB, which results in EM contrast after treatment with osmium (PMID:5962951, PMID:7962049). For proteomic analysis, live cells are exposed to hydrogen peroxide and a suitable substrate (such as biotin-phenol, biotin-arylazide or fluorescein-arylazide). HRP catalyzes the formation of short-lived radicals that covalently attach to proximal endogenous proteins, allowing their purification (PMID:22106087, PMID:24706754). For a comparison of HRP with other protein proximity labeling tools, see PMID:26355100, PMID:27667171, PMID:28387482, PMID:29125959.