ESR1 and breast carcinoma: As expected and in line with literature (Perou et al., 2000), most ERα‐positive primary breast cancers we studied also expressed FOXA1 (n = 73/81, 90%; staining percentage range 3–100%, mean 70%) and GATA3 (n = 63/81, 78%; staining percentage 1–100%, mean 58%).