PDCD1 and breast cancer: In response to the accumulating evidence on the important role of the tumor microenvironment (composed of stromal cells, immune cells, vasculature, extracellular matrix, and various secreted factors) in cancer genesis, growth and metastasis [8,9,10], the World Health Organization recommended histological assessment of the tumor milieu (in terms of tumor-infiltrating lymphocytes, fibrotic focus and Programmed Cell Death Protein 1/Programmed Cell Death Ligand 1 (PD-1/PD-L1)) expression for breast cancer classification [11].