• BCR-ABL, an abnormal gene produced by a specific chromosomal rearrangement or “translocation”, known as the Philadelphia chromosome. The altered BCR-ABL protein resulting from the Philadelphia chromosomal rearrangement is overexpressed (amplified) in most cases of chronic myelogenous leukemia (CML). Imatinib (Gleevac®) specifically targets or blocks (inhibits) the abnormal BCR-ABL protein, which is a type of protein known as a “tyrosine kinase receptor.”
• HER2-neu, a protein on the surface of cells that is overexpressed (amplified) in approximately 25 to 30% of breast cancers and in some cancers of the stomach and the junction where the esophagus meets the stomach (gastroesophageal junction). HER2 protein overexpression leads to uncontrolled cell growth and division, and HER2 positive cancers tend to grow faster and be more aggressive than HER2 negative cancers. Trastuzumab (Herceptin®) is an agent known as a monoclonal antibody that targets the HER2 receptors, which may help keep the cancer from growing.
• Mutations of the KRAS gene in advanced colorectal cancer have been found to be predictive of a poor response to agents that target the epidermal growth factor receptor (EGFR), such as cetuximab (Erbitux®). EGFR is often overexpressed in colorectal cancers. The KRAS gene is involved in many EGFR pathways, and it is thought that KRAS mutations may be a possible barrier to the effects of anti-EGFR agents. Such findings are important in guiding treatment decisions for patient with and without KRAS mutations.
• C-KIT is a protein (tyrosine kinase receptor) that is mutated and abnormally activated in cells in gastrointestinal stromal tumors (GIST). The agent imatinib (Gleevec®) is a tyrosine kinase receptor inhibitor, which blocks the C-kit receptors, inhibiting cell proliferation. (As noted above, imatinib is also used in the treatment of patients with CML caused by overexpression of the abnormal tyrosine kinase receptor protein BCR-ABL.
• Patients with non-small-cell lung cancer who have mutations in a specific region of the EGFR gene have had an improved response to treatment with gefitinib (Iressa®) compared to patients without such mutations.

Researchers have found that gefitinib interacts with the EGFR protein precisely in the region where the EGFR gene is mutated. Most patients with malignant melanoma, an aggressive skin cancer, have mutations of a gene called B-RAF. In those patients, the agent vemurafenib (Zelboraf®) inhibits B-RAF activity, inhibiting cell proliferation.