Cancer therapies that target specific genetic abnormalities in tumors have revolutionized treatment possibilities over the past two decades. While quality of life and survival are improved with targeted therapies, relapse is common due to the evolution of new tumor cells that are resistant to the targeted therapy. A new study reveals how lung tumors may develop drug resistance over time, pointing to a protein, called APOBEC3A, that could be a promising target. Results may help researchers develop new solutions for tumor resistance to targeted cancer therapies.