The pooled results showed that PD-L1 expression was increased in patients with positive lymph node metastasis (OR=1.59, 95% CI 1.06-2.39; P=0.02), higher histological grade (OR=1.68, 95% CI 1.37-2.06; P < 0.001), estrogen receptor (ER)-negativity (OR=0.24, 95% CI 0.42-0.06; P=0.008) and triple negative breast cancer (TNBC) (OR=1.70, 95% CI 1.24-2.33; P < 0.001). This evidence concerns the gene ESR1 and triple-negative breast carcinoma.