PDCD1 and cancer: Various keywords were utilized, such as “natural compounds” AND “cancer” AND “PD-1” or “PD-L1”, with more specific terms like “biological products” OR “biological” AND “products” OR “biological compounds” OR “natural” AND “compounds” OR “natural products” AND “cancers” OR “cancerated” OR “canceration” OR “cancerization” OR “cancerized” OR “cancerous” OR “neoplasms” OR “neoplasms” OR “cancer” OR “cancers” AND “PD-1” OR “PD-L1” OR “programmed cell death 1 receptor” OR “programmed cell death protein 1”.