Homologous recombination (HR) is a major repair mechanism for DNA double-strand breaks induced by ionizing radiation. A key step in HR is the formation of Rad51 nucleoprotein filaments on the single-stranded DNA generated from these breaks. We have shown that strict control of these filaments is essential, so that HR does not itself induce chromosomal rearrangements (eLife 2018, Cells 2021). In humans, functional homologs of control proteins are tumor suppressors. Thus, the control of HR appears to be as important as the HR mechanism itself. Our project involves the use of new molecular tools enabling a real breakthrough in the study of these controls. We will be using a functional fluorescent version of the Rad51 protein developed for the first time by our collaborators A. Taddei (Institut Curie), R. Guérois and F. Ochsenbein (I2BC, Joliot, CEA). This major advance will enable us to observe the influence of control proteins on DNA repair by microscopy in living cells. We have also developed highly accurate structural models of control protein megacomplexes in association with Rad51 filaments. This study also led to the identification of specific domains for each paralog protein, outside the structurally conserved Rad51-like core, that might define the specificity of each paralog proteins. We will use a multidisciplinary approach based on genetic, molecular biology, biochemistry, protein structure and live microscopy methods and yeast as model organism to study the consequences of the ablation of these specific domains. We will also search for proteins specifically binding these domains. Their identification would be crucial to understand the function of Rad51 paralog complexes and help to develop new therapeutic approaches.