The precise mechanism determining whether the labeling reagent or the Tyr residue on the protein undergoes radicalization, as well as the specific reaction intermediates involved, is still not fully understood and remains open to further investigation. However, in tyrosine modifications using tyramide, the radicalradical recombination pathway has been shown to predominate over other pathways between neutral Tyr residue and radical. Additionally, a report indicated that the reaction can proceed even under conditions where the radical species of the labeling reagent is generated electrochemically at potentials that do not directly radicalize Tyr. Several reagents have been reported for Tyr modification, including tyramide, phenylenediamine, N-methylurazole, and phenoxazine, while pyrazole has been shown to label phenylalanine (Phe) residues. In the catalytic mechanism, for instance, Ru catalysts can be photoexcited, and with the presence of oxidants such as oxygen or ammonium persulfate (APS), transition to a Ru(III) state. This Ru(III) species subsequently oxidizes the substrate via SET as it returns to its ground Ru(II) state. The use of Ru-based photocatalysts has long been known for Tyr-Tyr cross-linking reactions. More recently, it has become possible to label proteins by mimicking tyrosine residues with tyramide derivatives as labeling reagents. Tyramide conjugated with tags such as biotin (often referred to as "biotin phenol") enables visualization of labeled proteins and proteomics analysis through enrichment using avidin-beads. Additionally, methods have been explored for switching reactions using complexes with quencher molecules on DNA oligomers. Apart from tyramide, ethylenediamine-type labeling reagents have been utilized for radical modifications of tyrosine residues. This labeling reagent is also capable of labeling not only Tyr but also Cys, when free Cys residues are located near the catalyst binding site. Additionally, N-methylurazole has been developed as a radical-based labeling reagent specifically for labeling tyrosine residues in proximity to Ru complexes. The ligand structure of these catalysts has been examined from multiple perspectives, including the minimization of nonspecific adsorption to proteins, as well as enhancement of ligand binding with proteins. The importance of such proximity effects is described in the Supramolecular catalysis section as well.