To establish the functional importance of STAT3 as a downstream effector of active ATM in maintaining the tumorigenicity of cancer cells, we transduced a constitutively active (STAT3C) or a dominant-negative (STAT3DN) version of STAT347 into ATM-knockout MDA-MB-231 breast cancer cells and D456MG patient-derived glioma cells. The gene discussed is STAT3; the disease is breast cancer.