Protein Kinases (PK) comprise a large family of enzymes that mediate the transfer of a phosphate group from a donor molecule (usually ATP) to hydroxyamino acids. The enzymes fall into two broad classes, characterized with respect to substrate specificity: serine/threonine specific and tyrosine specific. Protein kinases play an essential role in many signaling pathways, and therefore have the potential to contribute to diseases ranging from cancer and inflammation to diabetes and cardiovascular disorders, not to mention cell growth, survival and many other biological functions. Most PK inhibitors act by competing with ATP for binding to the kinase. This is possible because of structural similarities between ATP and the inhibitors.