In order for multicellular organisms to function normally, it is necessary to assure an adequate cell communication. Signal molecules play the main role in cell signalling pathways. They bind to specific cell surface receptors transmitting the information about their presence to the cell interior on several different ways. This process, from the signal molecule recognition to physiological response formation, is termed signal transduction. One example of a molecular signal that could provoke a physiological response is epidermal growth factor (EGF). EGF is excreted as a response to tissue damage and stimulates cells growth and proliferation. EGF is a small protein that binds to a receptor tyrosine kinase specialised for EGF and induces its dimerization. Dimerization then generates cross-phosphorylation that activates intracellular signalling cascade. Activation of the transcription factors Ras/Raf/Mek/Erf leads to an altered gene expression. Cell signalling pathways are extremely complex and therefore prone to errors. This mainly refers to mutated gene that encodes a protein of a signalling pathway or it can be due to an increased EGF receptor expression. In both cases changes in the EGF signalling pathway occur. Most common manifestation of such disturbance is a malignant disease.