Hypoxic conditions can induce the transition of tumor cells from an epithelial phenotype to a mesenchymal phenotype and enhance radiation resistance through increased levels of hypoxia-inducible factor 1 (HIF-1). Cancer-associated fibroblasts (CAFs) induce epithelial-mesenchymal transition in tumor cells and secrete CXCL1, which inhibits the expression of the reactive oxygen species (ROS) scavenging enzyme superoxide dismutase 1. This leads to increased ROS accumulation after radiation, resulting in radiation resistance. The gene discussed is HIF1A; the disease is neoplasm.