Growth and development are dependent on the faithful duplication of cells. Duplication requires accurate genome replication, the repair of any DNA damage, and the precise segregation of chromosomes at mitosis; molecular checkpoints ensure the proper progression and fidelity of each stage. Loss of any of these highly conserved functions may result in genetic instability and proneness to cancer. Here we show that highly significant increases in chromosome missegregation occur in cell lines lacking the RAD51-like genes XRCC2 and XRCC3. This increased missegregation is associated with fragmentation of the centrosome, a component of the mitotic spindle, and not with loss of the spindle checkpoint. Our results show that unresolved DNA damage triggers this instability, and that XRCC2 and XRCC3 are potential tumour-suppressor genes in mammals.