Tyrosine phosphorylation refers to the addition of a phosphate group to the hydroxyl oxygen of the tyrosine amino acid residue in a protein. This process is catalyzed by specific kinases, such as protein tyrosine kinases （PTKs）, which transfer the gamma phosphate group of ATP to the tyrosine hydroxyl group. Tyrosine phosphorylation is involved in the regulation of protein function, activity, and localization. It plays a key role in signal transduction pathways, mediating cellular responses to extracellular stimuli.