Figure 3).

Mitogen-activated protein kinase (MAPK) intracellular signalling pathway. After binding of growth hormone (epidermal growth factor, platelet-derived growth factor, fibroblast growth factor-2, insulin-like growth factor-1) to its tyrosine kinase receptor, the receptor becomes activated as a protein tyrosine kinase (PTK). PTK undergoes autophosphorylation at multiple tyrosine residues, which interacts with signalling proteins, in turn activating Ras, a low molecular weight G-protein. Ras leads to activation of MAPK through protein kinase cascade. Thereafter, MAPK induces the transcription of proto-oncogenes (c-fos, c-myc, c-myb, b-myb, ras)