APEX2 is an engineered form of a soybean ascorbate peroxidase (APX1). It can be used as a genetically encoded tag for electron microscopy (EM) or as a proximity labeling tool for proteomic analysis. For EM, cells expressing APEX2 are fixed and then exposed to diaminobenzidine (DAB) and hydrogen peroxide. APEX2 catalyzes the hydrogen peroxide-dependent polymerization and local deposition of DAB, which results in EM contrast after treatment with osmium. This property can be used to study the ultrastructural location of a tagged protein of interest, or to to study the ultrastructure of an organelle or other subcellular structure. For proteomic analysis, APEX2 is genetically targeted to a protein complex or organelle of interest and live cells are exposed to hydrogen peroxide and biotin-phenol. APEX2 catalyzes the formation of a very short-lived biotin-phenoxyl radical which covalently attaches to proximal endogenous proteins, allowing their purification using streptavidin beads (adapted from PMID:25419960). APEX2 acts as a monomer and contains the following mutations relative to wild-type soybean APX1: K14D, W41F, E112K, A134P (PMID:25419960). For a comparison of APEX2 with other protein proximity labeling tools, see PMID:30774936.