BackgroundWe previously demonstrated a progressive loss of the anti-human epidermal growth factor receptor 2 (HER2) CD4+ T-helper type 1 (Th1) response during HER2pos breast tumorigenesis. This loss is associated with residual disease following neoadjuvant therapy and increased risk of recurrence. In this study, we assessed the fate of anti-HER3 Th1 immunity during breast tumorigenesis.