PDCD1 and neoplasm: Pembrolizumab, the only FDA-approved immune checkpoint inhibitor (ICI) for prostate cancer, targets PD-1 but is specifically approved for metastatic castration-resistant prostate cancer (mCRPC) patients with high microsatellite instability (MSI-H), deficient mismatch repair (dMMR), or a tumor mutational burden (TMB) of at least 10 mutations per megabase, particularly as a subsequent treatment after docetaxel and hormonal therapy failure.