APEX2 is an engineered ascorbate peroxidase that has been developed as a powerful molecular tool for proximity labeling and electron microscopy applications, rather than functioning as a DNA repair enzyme like its namesake suggests. The protein generates biotin-phenoxyl radicals in the presence of hydrogen peroxide and biotin-phenol, which covalently tag nearby proteins within a nanometer radius 1. This proximity labeling occurs within one minute of substrate addition, providing temporal precision for capturing transient protein interactions 1. APEX2 was evolved from the original APEX enzyme to have significantly enhanced catalytic efficiency, enabling applications requiring lower expression levels 2. The system has been successfully applied to map subcellular proteomes of specific organelles including mitochondria and endoplasmic reticulum 21, create spatial maps of RNA localization 3, and identify context-dependent protein interactions in disease states 45. APEX2 can be targeted to specific RNAs using MS2-MCP or CRISPR-Cas13 systems to map RNA-protein interactions 6. The technology enables multiplexed quantitative proteomics studies and has become widely applicable for investigating spatial organization of cellular components 7.