To uncouple alterations in uracil that resulted from primary tumour growth from those attributable to metastasis, we orthotopically transplanted mammary tumour fragments obtained from K14-Cre; Trp53fl/fl (KP) mice into the 4th mammary fat pad of Upp1+/+ and Upp1−/− recipient mice and assessed circulating uracil levels when primary tumours were measurable but metastatic dissemination was not. This evidence concerns the gene KRT14 and breast cancer.