Kinases are enzymes that catalyze the transfer of phosphate groups from ATP. Receptor tyrosine kinases (RTKs) are tyrosine kinases on the cell surface that are activated through binding of growth factors. Receptor tyrosine kinsases play a role in signaling pathways that govern normal cellular processes such as proliferation, migration, metabolism, and differentiation. Examples of RTKs include: platelet-derived growth factor receptor (PDGFR), vascular endothelial growth factor receptor (VEGFR), stem cell factor receptor (Kit). In normal cells, RTK activity is tightly controlled. In some cancerous conditions, abnormalities such as mutations can lead to abnormal activation of RTKs resulting in increased cellular proliferation and growth, prevention of apoptosis (cellular death) as well as increased angiogenesis and metastasis. The result of RTK dysfunction is phosphorylation of the kinase in the absence of an appropriate signal and constitutive signaling and abnormal promotion of cell growth and survival. These conditions promote tumor development, growth and progression.