Hence, in this section, we first introduce the PD-1/PD-L1 axis and then point out: (1) the level of PD-L1 expression in HER2+ BC, (2) the association of PD-L1 expression to disease progression and response to therapy, and (3) some of the factors that are linked to the overexpression of PD-L1. The gene discussed is CD274; the disease is breast cancer.