Additionally, as BRCA1-associated breast tumours are typically oestrogen receptor (ER) negative we explored the correlation with ESR1. ER negative tumours had significantly greater expression variability (Fig. 2D, Brown–Forsythe test p = 2.19 × 10−106) and tumours with low ESR1 expression had a greater range of expression. Here, ESR1 is linked to breast neoplasm.