This paper basically conducted serial biopsies to determine the mechanisms of resistance to EGFR inhibitors. In 3 patients, resistance to EGFR inhibitors was lost after a patient had been off the medication for a while and the patients were able to be retreated with EGFR TKIs. This illustrates a concept from evolutionary biology - when a drug is around, resistant cells gain an advantage and are able to thrive. When we take away the drug, the original cell type (which may have been more "fit" than the resistant cells) begins to take over again. Although a lot of us have thought that this was possible, Dr. Sequist's findings are a very compelling illustration of this principle. In 2 cases, the patients had developed T790M as the mechanism of resistance and then this mutation was no longer found after the patients had been off EGFR therapy for a while.
Another very interesting finding from the paper is that in 14% of patients, the resistant tumors had transformed from NSCLC into SCLC. Of course, the total number of patients was small (37) - 14% seems like a high number to me for this phenomenon.