Given our previous data showing that UPP1-expressing neutrophils from tumour-bearing mice export uracil into the extracellular microenvironment (Fig. 2F), and the consistent finding that increased uracil is detected in the circulation of mice and humans with metastatic cancer (Fig. 1B–F), we hypothesised that increased extracellular uracil generated by UPP1-expressing cells may influence ECM deposition by fibroblasts. This evidence concerns the gene UPP1 and metastatic malignant neoplasm.