In this work, we investigated PD-L1 expression and the underlying mechanisms in cisplatin (CDDP)-resistant human non-small cell lung cancer cells (A549/CDDP), doxorubicin (ADR)-resistant human breast cancer cells (MCF7/ADR) and human hepatocellular carcinoma cells (HepG2/ADR). The gene discussed is CD274; the disease is breast cancer.