sAPEX 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 sAPEX are fixed and then exposed to diaminobenzidine (DAB) and hydrogen peroxide. sAPEX 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, sAPEX is genetically targeted to a protein complex or organelle of interest and live cells are exposed to hydrogen peroxide and biotin-phenol. sAPEX catalyzes the formation of short-lived biotin-phenoxyl radicals which covalently attach to proximal endogenous proteins, allowing their purification using streptavidin beads. sAPEX contains the following mutations relative to wild-type soybean APX1: K14D, W41F, E112K (K14D and E112K are monomerizing mutations) (PMID:23371551).