Some proteins are regulated via phosphorylation (the addition or removal of a phosphate group) at a specific amino acid group (either a serine, threonine, or tyrosine residue).  Phosphorylation is a reversible  process; protein kinases add phosphate groups, while phosphatases remove them.  Protein kinase activity is of particular interest, since oncoproteins (cancer-causing proteins), the products of oncogenes often have phosphorylating activity.