Triple negative breast cancer (TNBC), the most aggressive and metastatic subtype of breast cancer, is one of the major causes of cancer death in women. TNBC also has lower survival rates for primarily local cancers. Loss of hormone receptors and the lack of HER2 overexpression in TNBC limit treatments to cytotoxic therapies such as radiation, a key clinical strategy for 10-15% of breast cancer patients. Hypoxia is a major cause of resistance to radiotherapy, chemotherapy and even immunotherapy. Thus, identifying mechanisms to suppress the hypoxia stress response and increase the sensitivity of TNBC tumors to therapy remains a top clinical priority. Upon hypoxic stress, cancer cells initiate a transcriptional program that enables them to survive and migrate from an inhospitable microenvironment. While hypoxia-inducible factors (HIFs) are generally considered the main effectors, growing evidence suggests the hypoxia cellular response is much more complex and requires coordinated signaling with other stress response factors. One promising candidate is BACH1, a transcription factor that is induced by hypoxia and represses transcription of genes involved in heme oxidation and anti-oxidant production. Based on preliminary results, we now hypothesize that BACH1 is stabilized by hypoxia and functions as a key inducer of the cellular hypoxia response in TNBC cells leading to abnormal leaky vasculature that contributes to intratumoral hypoxia and radiation resistance. Specifically, we plan to: 1. Determine whether BACH1 is regulated by oxygen and induces a transcriptional hypoxic stress response in TNBC cells; 2. Determine whether BACH1 promotes angiogenesis, leaky vasculature and hypoxia in TNBC tumors; and 3. Determine whether BACH1 depletion sensitizes TNBC tumors to radiation. We propose that targeting BACH1 represents a unique strategy for increasing tumor oxygenation to improve the efficacy of cytotoxic, standard-of-care therapies such as radiation. Normalizing vasculature to suppress leakiness should also facilitate drug delivery to tumors. Since BACH1 can be targeted by an FDA approved drug either alone or in combination with HIF inhibitors, the proposed work could lead to a clinical trial in breast cancer and other cancer patients whose treatment involves radiation therapy.