CD274 and breast cancer: Our observation that both PD-L1 and CMTM6 were highly expressed in the breast cancer triple-negative MDA-MB-231 cells (Fig. 5), a cell line widely used in PD-L1 studies, led us to use these cells to examine the half-life of PD-L1 after stimulation of the cells with REIC/Dkk-3.