Epidermal Growth Factor Receptor — Cell Growth Protein

The epidermal growth factor receptor is a member of the ErbB family of receptor tyrosine kinases. The protein structure consists of three key domains: an extracellular ligand-binding domain exposed outside the cell, a single transmembrane segment, and an intracellular tyrosine kinase domain. Under normal physiological circumstances, specific proteins called ligands—such as epidermal growth factor (EGF) or transforming growth factor-alpha (TGF-alpha)—bind to the external domain, prompting two receptors to link together (dimerise).

This dimerization activates the internal tyrosine kinase enzyme, initiating a cascade of downstream biochemical signalling pathways, including RAS-RAF-MEK-ERK and PI3K-AKT-mTOR. These internal signals instruct the cell to replicate its DNA, grow, resist programmed cell death (apoptosis), and promote new blood vessel formation. When the EGFR gene acquires activating mutations or undergoes amplification, this signalling pathway becomes continuously turned on, allowing cells to multiply unchecked and contributing significantly to the development and progression of various malignant tumours.

Identifying abnormalities in the EGFR protein or its underlying gene has fundamentally changed modern cancer diagnosis and therapy. Rather than relying solely on the anatomical location of a tumour, oncologists use EGFR status to define the molecular identity of the disease. In patients whose cancers are driven by EGFR alterations, standard therapies can be substituted or combined with targeted treatments tailored to silence this specific receptor. This precision approach often results in better disease control, improved progression-free survival, and fewer non-specific toxicities compared to unselected traditional treatments.

EGFR status is assessed using tissue collected during surgery or biopsy, or via circulating tumour DNA from blood samples (liquid biopsy). Polymerase chain reaction (PCR) assays and next-generation sequencing (NGS) gene panels identify specific activating mutations in exons 18 to 21 of the EGFR gene. Immunohistochemistry (IHC) is utilised to measure how much EGFR protein is expressed on the surface of tumour cells, while fluorescence in situ hybridisation (FISH) can determine whether the cell contains extra copies of the EGFR gene (gene amplification).

Aberrant EGFR signalling is implicated in several major cancers. Activating EGFR mutations are most prominently detected in non-small cell lung cancer (NSCLC), especially adenocarcinomas in non-smokers and female patients. Overexpression or amplification of the wild-type EGFR protein occurs frequently in squamous cell carcinomas of the head and neck and in colorectal carcinoma. It is also found in glioblastoma, a primary brain malignancy where a distinct variant known as EGFRvIII is frequently observed, as well as in subsets of pancreatic and oesophageal cancers.

Detecting an EGFR abnormality provides direct guidance for oncologists selecting systemic therapies. Patients with sensitising EGFR mutations in lung cancer are treated with oral tyrosine kinase inhibitors (such as osimertinib), which block the internal enzymatic activity of the receptor. For cancers driven by EGFR overexpression without downstream mutations, such as wild-type RAS colorectal cancer or head and neck cancer, intravenous monoclonal antibodies (such as cetuximab) are used to block the external receptor domain. Regular monitoring tracks whether new secondary mutations develop, indicating the need to switch therapeutic agents.