CD274 and neoplasm: First, the PD-L1 expression levels of the test cell lines were determined, and the expression in descending order was HCC1954, HCC827, MDA-MB-231, HT29, and CHO-K1, in which HCC1954, HCC827, and MDA-MB-231 should belong to PD-L1-positive tumor cell lines (Additional file 1: Table S2).