Yashpal Rawal,et al.

Homologous recombination (HR) fulfils a pivotal role in the repair of DNA double-strand breaks and collapsed replication forks1.HR depends on the products of several paralogues of RAD51,including the tetrameric complex of RAD51B,RAD51C,RAD51D and XRCC2 (BCDX2)2.BCDX2 functions as a mediator of nucleoprotein filament assembly by RAD51 and singlestranded DNA (ssDNA) during HR,but its mechanism remains undefined.Here we report cryogenic electron microscopy reconstructions of human BCDX2 in apo and ssDNA-bound states.The structures reveal how the amino-terminal domains of RAD51B,RAD51C and RAD51D participate in inter-subunit interactions that underpin complex formation and ssDNA-binding specificity.Single-molecule DNA curtain analysis yields insights into how BCDX2 enhances RAD51-ssDNA nucleoprotein filament assembly.Moreover,our cryogenic electron microscopy and functional analyses explain how RAD51C alterations found in patients with cancer3-6inactivate DNA binding and the HR mediator activity of BCDX2.Our findings shed light on the role of BCDX2 in HR and provide a foundation for understanding how pathogenic alterations in BCDX2 impact genome repair.