In this retrospective study, we further dissect the heterogeneity of HER2-positive breast cancer by evaluating the characteristics of the BCR and TCR repertoires in the context of two phase III clinical trials, namely NeoALTTO and CALGB 40601, evaluating neoadjuvant paclitaxel with trastuzumab and/or lapatinib in women with HER2-positive breast cancer. The gene discussed is ERBB2; the disease is breast cancer.