ERBB2 and breast cancer: Earlier studies found strong evidence for increased APOBEC3B activity and mutagenesis in a subset of breast cancers [7, 20, 21, 61] and with APOBEC signature enrichment in the HER2 breast cancer subtype and in triple negative breast cancer (TNBC) [6, 62]; however, a study of breast cancer cell lines found generally low levels of APOBEC3A expression [29].