Cellular immunotherapies, in which a patient’s immune system is genetically engineered to target cancer cells, are revolutionizing cancer treatment. However, these engineered cells do not always persist long-term in patients, which can lead to disease relapse. I propose that the DNA structure of these engineered immune cells changes over time, ultimately impairing the expression of receptors which target the cancer cells, and that these changes can lead to treatment failure. I will examine genetically engineered immune cells from patients receiving them for cancer treatment for this phenomenon, and correlate the DNA structural changes with engineered receptor expression and therapeutic response.