This impairment of barrier function during tumorigenesis may contribute to IL-9 effects in tumour development, as gut bacteria favour protumoral mucosal Th17 signatures and local inflammation in colorectal neoplasias.45 Mucosal inflammation in turn may further induce the expansion of microorganisms with genotoxic capabilities which favour tumour progression in CRC.4 Here, IL9 is linked to neoplasm.