Bacteria‐derived Trp metabolites such as trans‐3‐indolacrylic acid (IDA), indole‐3‐aldehyde (I3A), indole‐3‐carboxylic acid (ICA) and indole acetic acid (IAA) are defined as AhR ligands that play important roles in cellular homeostasis, host immunity and tumour progression, suggesting that AhR acts as a mediator between Trp‐metabolising bacteria and the host. The gene discussed is AHR; the disease is neoplasm.