Importantly, although primary tumours grew at similar rates (Supplementary Fig. 6b), mice injected with Il1b−/− fibroblasts had a significantly smaller lung metastatic burden compared to mice injected with WT fibroblasts (Fig. 5f–h), indicating that CAF-derived IL-1β contributes to breast cancer metastasis even in an IL-1β proficient microenvironment. The gene discussed is IL1B; the disease is breast cancer.