Introduction

Protein phosphorylation was first discovered by Fischer and Krebs in the mid-1950s (1), and it has been generally accepted that reversible protein phosphorylation regulates virtually every physiological event in mammalian cells. There are approximately 518 protein kinases in human cells. Among them, 89 are tyrosine kinases (2). Phosphorylation by protein tyrosine kinases is crucial to the control of development and growth of multi-cellular organisms. Deregulation or mutation of tyrosine kinases in human cancers has been repeatedly reported in the literature (3). About a quarter of tyrosine kinases were originally discovered as oncogenes, and represent the largest family of oncogenes. Tyrosine kinases are classified as receptor and non-receptor tyrosine kinases. Both classes of tyrosine kinases catalyze the addition of a phosphoryl group on a tyrosine residue but at different locations within the cell – whereas receptor tyrosine kinases (RTKs) are transmembrane proteins, non-receptor tyrosine kinases (NRTKs) are intra-cellular. At present, there are 57 known RTKs in mammalian cells classified into about 20 families, whereas 32 are NRTK, classified into approximately 10 families (Table 8.1).