Consistent with a previous study [19], which found a significant inverse association between breast cancer PRS and TSMC in breast tumor samples for ER+ patients, we found inverse associations between the somatic mutation counts of APOBEC-related signatures and breast cancer PRS (both overall and ER+ specific) in breast tumors, though the associations were slightly above the significance threshold (p = 5.08 × 10−5 for ER+, p = 1.14 × 10−4 for overall). This evidence concerns the gene ESR1 and breast neoplasm.