FN1 and neoplasm: Addition of 30 μM uracil (the concentration of uracil associated with high metastatic burden in MMTV-PyMT tumour-bearing mice (Fig. 1C)) enabled mouse fibroblasts to deposit ECM with increased fibronectin content in both 2D and 3D microenvironments, whereas 10 μM uracil (associated with little or no metastatic burden) was less effective in these regards (Figs. 5C and EV5B).