Protein kinases are enzymes that modify other proteins by chemically adding phosphate groups to them (phosphorylation).

Phosphorylation usually results in a functional biological change of the target protein by changing enzyme activity, cellular location, or association with other proteins.

Protein kinases are a major factor in regulating most of the cellular control and signal transduction pathways. Protein kinase modifying reagents are therefore important both in cancer research and as drug candidates.