UPP1 and breast cancer: Indeed, mice which lack the gene for Upp1 (Upp1−/− mice), display increased levels of uridine and decreased levels of uracil in tissues and serum (Appendix Fig. S3A–B) (Cao et al, 2005), whilst mammary tumours from MMTV-PyMT mice displayed reduced uracil, but unaltered uridine levels (Appendix Fig. S3C).