A study reveals why cancer cells become resistant to drugs, why treatment no longer works and why cancer recurs. The study concluded that the development of drug resistance is a fait accompli. The timing of cancer recurrence is simply determined by the timing of the proliferation of cancer cells carrying the mutated gene. Long-term remission of advanced cancer using a single targeted drug is almost impossible to achieve. The study included 28 patients with advanced colon cancer treated with panitumumab, 24 of whom had tumors that were KRAS wild-type prior to treatment and four KRAS mutant patients served as controls. Every 4 weeks during the first few treatments, the researchers took blood samples from the patients, for a total of 169 blood draws. As a result, nine of the original 24 KRAS wild-type patients (38%) had detectable KRAS mutations in their blood within 5-7 months of treatment, and three patients had imaging scans showing metastatic tumor growth after KRAS mutations were detected. Subsequently, they collaborated with Martin Nowak’s research team at Harvard University to calculate the likely time of origin of KRAS mutations using mathematical models, which determined that KRAS mutations were present prior to treatment with panitumumab.