A polymorphism in Toll-like receptor 5 (TLR5R392X), which prevents the ability to respond to flagellin, occurs in ∼7% of humans. Mice without this polymorphism respond to flagellin from commensal bacteria; this leads to increased systemic interleukin-6 (IL-6) levels and immunosuppression, resulting in the progression of existing tumours. In tumour-bearing mice expressing TLR5-R392X, IL-17 was upregulated and tumour growth was more restrained; this difference was abrogated when commensal bacteria were ablated. Patients with ovarian or breast cancer also had the different cytokine profiles according to the presence of the TLR5R392X polymorphism.