As endogenous uracil is transformed into dihydrouracil by DPD, an elevated pre-treatment uracil concentration in plasma (>14 or 16 ng/ml) is predictive for severe fluoropyrimidine-related adverse events.15, 16, 17 Genotyping for certain variants in the gene coding for DPD, DPYD, was also found to be predictive for DPD deficiency and therefore increased risk of severe adverse events.3 The gene discussed is DPYD; the disease is hyperinsulinemic hypoglycemia, familial, 4.