CD274 and breast cancer: PD-L1 is expressed in only approximately 20% of TNBC patients (Mittendorf et al., 2014) and 18% of HER2-positive breast cancer patients (Hou et al., 2018), and approximately 50%–80% of patients with tumors show a limited response to ICB monotherapy, with no clinical benefit (Fukumura et al., 2018).