Interestingly, uridine (which DESI-MS revealed to have a high relative abundance in the tumor epithelium of pre-NACT PR samples), which serves as a substrate for cytidine 5-prime triphosphate synthetase (CTPS1), was highly abundant in pre-NACT PR samples (logFC 0.97, LIMMA p = 0.008) (Supplementary Table 18). Here, CTPS1 is linked to neoplasm.