SMAD4 and gastric cancer: We noticed that the basal protein levels of PD-L1 are higher in NCI-N87 and AGS cells than other gastric cancer cells as shown in Fig. 1a, which may attribute to the specific genomic mutations harbored by the cells with SMAD4 and TP53 mutations in NCI-N87 cells and CDH1, CTNNB1, KRAS and PIK3CA mutations in AGS cells.