Circulating uracil levels were also unchanged with age in FVB/N mice (Fig. EV1H), and age did not significantly contribute to metastatic burden in this cohort of MMTV-PyMT mice (Fig. EV1I), suggesting that UPP1 activity in the primary tumour is not responsible for increasing uracil levels in the circulation of mice with lung metastasis, and that age is not a factor in dictating serum uracil concentration at tumour endpoint in the MMTV-PyMT model of mammary cancer. Here, UPP1 is linked to breast cancer.