PCNA and head and neck squamous cell carcinoma: Our principal findings demonstrate that (1) HNSCC tumorigenesis is associated with a significant decrease in intratumoral microbiota diversity and distinct shifts in microbial composition; (2) specific bacterial genera and species are significantly enriched or depleted in tumors; (3) the altered microbiota is correlated with proliferative activity, as indicated by PCNA expression; and (4) predictive functional profiling suggests that the intratumoral microbiota may influence host metabolic pathways, potentially contributing to tumor microenvironment remodeling and HNSCC pathogenesis.