ERBB2 and breast cancer: ERBB2 (HER2) amplifications are predictive of clinical benefit with anti-HER2 therapies, such as neratinib.62,63ESR1 mutations arise in patients treated with aromatase inhibitors and modify endocrine therapy response.64 Microsatellite instability (outside of Lynch syndrome) and AKT and NTRK fusions are rare in BC.