PDCD1 and breast cancer: BC has previously been considered an immunogenically quiescent or “cold” tumour due to low lymphocyte infiltration, low mutational burden, and limited response rates to anti-programmed cell death 1 (PD-1)/programmed cell death ligand 1 (PD-L1) monotherapy; however, the identification of tumour infiltrating lymphocytes and other immune infiltrates in BC has led to the application of immunotherapies for the disease [15,16,17].