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Figure 4. Overview of how immune checkpoint inhibitors work in the context of prostate cancer. PD-L1 binds to PD-1, preventing T cells from killing tumor cells; blocking PD-L1 or PD-1 allows T cells to kill tumor cells. MSI-H (microsatellite instability) or MMR (mutational changes in the mismatch repair)-deficient prostate cancer cells, increased PD-L1 expression, increased tumor mutational burden (TMB), and CDK12 mutations contribute to a favorable response to immune checkpoint inhibitors. In some patients with MSI-H or MMR-deficient prostate cancer, ICI drugs that target PD-1 or PD-L1, such as pembrolizumab and nivolumab, have been proven to be effective.