This subsection introduces methods for oxidative protein labeling mediated by electron transfer reactions between photocatalysts and substrates (Scheme 3A, 3B). In this mechanism, the activated photocatalyst extracts an electron from the substrate, generating a radical species. Based on the oxidation potential of the photocatalyst, radicals can form on both labeling reagents and tyrosine (Tyr) residues of the protein. Due to high reactivities of these radical species in general, rapid bond formation often follows the excitation process. With the loss of an additional electron and proton, a reaction proceeds-in which, formally, two electrons and two protons are removed-resulting in the formation of either C-C or C-N bonds.