To examine whether the growth-suppression activity of fibroblast-secreted IL-25 protein was mediated by IL-25R (IL17RB) signalling in mammary tumour cells, we knocked down the expression of IL-25R in metastatic mammary carcinoma cells (MDA-MB-231 cells) using IL-25R-specific small interfering RNA (siRNA). The gene discussed is IL25; the disease is breast cancer.