ESR1 and breast cancer: Also 9p21 deletion was significantly associated with unfavorable tumor features, including histological grade (p < 0.0001), presence of lymph node metastases (p = 0.0063), ER/PR negativity (p < 0.0001 for ER and p = 0.0006 for PR), and amplifications of HER2 (p = 0.0078) in all breast cancers.