PTKs are enzymes that transfer the y-phosphate group of ATP to tyrosine residues on target substrate proteins. They are a large superfamily of enzymes comprising both transmembrane-spanning receptors with an intrinsic tyrosine kinase activity in their cytoplasmic domains and subfamilies of cytoplasmic tyrosine kinases such as (JAK) families.

Tyrosine phosphorylation is a covalent modificaiton of proteins. It provides a rpaid mechanisms of modifying the enzymic activity of target proteins. In addition, it can modify the ability of target proteins to act as adaptor molecules to recruit other signaling molecules. For example, the tyrosine phosphorylation of receptors or signaling molecuels creates docking sites that allow protein-protein interactions leading to recruitment of downstream signal transducers. Signal transducers are recruited to these phosphorylated tyrosines by protein-protein interactions domains, called WSH2 domains, contained with the sequence of many siganl transducers. SH2 domains comprise about 100 amino acids and spcifically recognize a phosphotyrosime plus the 3 amino acides immediatley C-terminal to that phosphotyrosine.

Tyrosine phosphorylation is reversible by the action of protein tyrosine phosphatases.