DPYD and hyperinsulinemic hypoglycemia, familial, 4: Given the alreadynarrow therapeutic window of 5-FU, DPD deficiency results in exaggerated 5-FU–related toxicities, including neutropenia, mucositis,stomatitis, diarrhea, skin rash, neurologic toxicities, and even death (Ciccolini et al., 2006; Saif et al., 2007).