FN1 and neoplasm: To understand whether UPP1 might influence the ECM-driven pre-metastatic niche priming, we used immunofluorescence to evaluate fibronectin deposition in the lungs of mice that were either wild-type or knockout for Upp1. This indicated that lung fibronectin levels were significantly decreased in MMTV-PyMT Upp1−/− tumour-bearing mice, but not by UPP1 knockout in non-tumour-bearing FVB/N animals (Figs. 5A,B and EV5A).