A protein kinase is a kinase enzyme that modifies other molecules, mostly proteins, by chemically adding phosphate groups to them (phosphorylation) to regulate the majority of cellular pathways, especially those involved in signal transduction. Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins. Of the 518 known kinases, the most successful class for drug targeting is the tyrosine kinase family consisting of 90 distinct and diverse members. Abnormal expression of PTK usually leads to cell proliferation disorders, and is closely related to tumor invasion, metastasis and tumor angiogenesis. More recently, PTKs play a pivotal role in inflammatory diseases such as idiopathic pulmonary fibrosis.

The Tyrosine Kinase Inhibitors Library by TargetMol, containing 1016 tyrosine kinase inhibitors, can be used for research in tyrosine kinase signaling, and drug screening for related diseases.