ESR1 and breast carcinoma: In an American study (n = 75) comparing MpBC to other breast cancer subtypes PD-L1 expression (≥ 2 staining intensity in ≥ 5% of tumor cells) was 46% in MpBC, 6% in ER-positive cases, 6% in HER2-positive and 9% in TNBC [26].