In this study, bioinformatic analysis based on The Cancer Genome Atlas (TCGA) Breast Cancer dataset was performed to determine whether ER downregulates the expression of PD-1/PD-L1, reduces the infiltration of CD8+ T cells, and affects the immune microenvironment by decreasing Th17 cell infiltration and inhibiting IL-17 signaling in breast cancer. The gene discussed is PDCD1; the disease is breast cancer.