Human genome encodes 58 receptor type protein tyrosine kinases (RTKs) and 32 non-receptor type protein tyrosine kinases (Robinson et al., 2000) (Fig. 1).
Each RTK contains a glycosylated extracellular ligand-binding domain with different sequence motifs, a single transmembrane portion and an intracellular domain with Tyr kinase activity that catalyzes the transfer of the γ-phosphate of bound ATP to the C-terminal domains and the Tyr residues of exogenous substrates (Blume-Jensen & Hunter, 2001). 48 human proteins have a kinase-like domain
that lacks at least one of the conserved catalytic residues; these proteins are therefore predicted to be inactive and have been termed pseudokinases (Boudeau et al., 2006).
RTKs are structurally diverse: 20 subfamilies/classes have been described so far (Lemmon and Schlessinger, 2010), and the members of a subfamily often bind common or similar ligands:
1) ErbB or Epidermal growth factor receptor subfamily
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Receptor Tyrosine Kinases (RTKs)