May 2024—Immunotherapy has revolutionized cancer treatment by recruiting the patient’s immune system to detect and destroy cancer cells. Immunotherapy often involves immune checkpoint blockade (ICB) agents, which target negative regulators of T-cell activation, such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), or programmed death-ligand 1 (PD-L1). Although ICB is used to treat a variety of cancer types, patients’ response to therapy is often unpredictable, and biomarkers such as tumor mutation burden, mismatch repair deficiency, and IHC for PD-L1 have limitations for assessing ICB response. Consequently, there is great interest in discovering additional biomarkers that will improve the ability to predict clinical response to ICB. Recent studies have explored the hypothesis that there may be a correlation between a person’s gut microbiome and therapeutic response.