EGFR: Epidermal growth factor receptor. A protein found on the surface of cells to which epidermal growth factor (EGF) binds. When EGF attaches to EGFR, it activates the enzyme tyrosine kinase, triggering reactions that cause the cells to grow and multiply. EGFR is found at abnormally high levels on the surface of many types of cancer cells, which may divide excessively in the presence of EGF. The drug Iressa attaches to EGFR and thereby inhibits the attachment of EGF and stops cell division. The gene for EGFR is on chromosome 7p12.3-p12.1. The EGFR molecule has 3 regions -- one projects outside the cell and contains the site for binding EGF; the second is embedded in the cell membrane; and the third projects into the cytoplasm of the cell's interior. EGFR is a kinase that attaches phosphate groups to tyrosine residues in proteins. EGFR is also known confusingly as ErbB1, ErbB, oncogene ErbB, and HER1.