ESR1 and breast cancer: Because a range of breast cancer subtypes were present on the tissue microarrays, we were able to determine that the expression of peak 13741 was positively associated with low tumor grade (P = 6.1 × 10-8), estrogen receptor (ER) expression (Figure 4c, top panels, P = 6.4 × 10-15) and progesterone receptor (PR) expression (P = 6.8 × 10-14) in breast cancers and showed no significant association with metastasis or human epidermal growth factor receptor 2 (HER2) expression (Table 1).