The invention relates to the classification of breast and ovarian tumours, and in particular to the use of particular rearrangement signatures to identify tumours as deficient in homologous recombination repair (HR-deficient). The inventors have identified particular chromosomal "hotspots" of recombination in breast and ovarian cancers which permit the homologous recombination repair status of a cancer to be assessed by determining the presence of recombination events within those specific hotspots, rather than by analysing the entire cancer genome for the presence of rearrangement signatures as a whole.
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