APOBEC3B and breast cancer: Among the 13 signatures previously identified as operative in primary breast cancer, the most represented signatures in the metastatic samples were signature 1, related to aging; signatures 2 and 13, related to APOBEC3B activity; signature 3, associated with failure of DNA double-strand break-repair by homologous recombination; and signature 6, associated with defective DNA mismatch repair (Fig 5).