Normally, cells in the body divide in an orderly way. However, in cancer cells, this normal process of cell division becomes abnormal and allows the cancer cells to grow in a rapid, unregulated way. In some patients with lung cancer, this abnormal and rapid growth in the cancer cells is drive by a specific change in a gene called the Epidermal Growth Factor Receptor (EGFR). This change in the EGFR gene in cancer cells is called a mutation. Patients with lung cancer harboring a mutation in EGFR can be treated with specific drugs called EGFR inhibitors. However, even though these drugs can be very effective, after a period of time, most EGFR tumors will develop resistance to this treatment, most often because of a second mutation in EGFR called T790M.