ESR1 and breast cancer: Similarly, Sabatier et al. evaluated the expression of PD-L1 mRNA in 45 breast cancer cell lines and 5454 breast cancer cases [1], and found that higher PD-L1 mRNA expression was associated with larger tumor size, higher histological grade, ER and PR negativity, HER-2 positivity, high proliferation, and the basal and HER-2 subtypes (known markers of a poor prognosis).