UPP1 (Uridine Phosphorylase 1): UPP1 is an enzyme involved in the pyrimidine salvage pathway, converting uridine into uracil and ribose-1-phosphate. It plays a crucial role in nucleotide metabolism, supporting DNA and RNA synthesis as well as other cellular functions. Its activity is especially important in chemotherapy, as it affects the activation and efficacy of certain anticancer drugs.