<qa>  What is the EGFR tyrosine kinase activated byEpidermal growth factor receptor EGFR is expressed or over-expressed in many human solid tumors and plays an important role in progression to invasion and metastases. The EGFR tyrosine kinase is activated by binding of a variety of ligands to the external domain. Autophosphorylation by the EGFR tyrosine kinase initiates a signaling cascade that feeds downstream cell cycle control machinery regulating cell proliferation, and these reactions are a major component in growth factor-induced proliferation of cancer cells. 