Breast cancer is the first type of cancer in women worldwide, and Europe has the highest rate of diagnosis. Chemoradiotherapy associated with surgery is part of standard breast cancer clinical care, but tumors can relapse, often because cancer cells acquire intrinsic chemoradioresistance. Our project aims to test the possibility that metabolic alterations are responsible for the acquired radioresistance of breast cancer cells to X-rays. Based on previous observations of our team in head and neck squamous cell carcinoma cells the focus will be set on mitochondria, as these organelles control DNA repair (ATP generation), cell proliferation (biosynthesis) and apoptosis (cytochrome c, superoxide and Ca2+ release). Because it is not protected by histones and has limited repair capacities, mitochondrial DNA (mtDNA) also potentially constitutes a main target of radiotherapy, which will be investigated together with mitochondrial turnover (mitophagy and mitochondrial biogenesis).