TYMS and hyperinsulinemic hypoglycemia, familial, 4: Considering the large variability in non-functional DPYD alleles reported among the different races and ethnicities, other enzyme variations associated with severe 5-FU toxicities (e.g., TYMS, ORPT), and the associated costs, prospective testing may not be feasible in patients receiving 5-FU therapy and may not accurately predict DPD deficiency in all patients tested [1,12,13,15,28].