The aim of this retrospective, historical cohort study was to analyze the relationship between ALDH1A1, programmed death ligand 1 (PD-L1) and tumor-infiltrating lymphocytes (TILs), assessed through immunohistochemistry, in triple negative (TN) and human epidermal growth factor receptor 2-positive (HER2+) breast cancer tumors, and its association with clinicopathological characteristics and survival. The gene discussed is ERBB2; the disease is breast cancer.