UPP1 (Uridine Phosphorylase 1): UPP1 is an enzyme involved in the pyrimidine salvage pathway and converts uridine into uracil and ribose-1-phosphate. It plays a crucial role in nucleotide metabolism and supports the synthesis of DNA and RNA as well as other cellular processes. Its activity is particularly important during chemotherapy, as it influences the activation and effectiveness of certain anticancer drugs.