DPYD and dihydropyrimidine dehydrogenase deficiency: Although total DPD deficiency is rare in adults, about 2–3% of the population has a low PBM–DPD enzyme level and, thus, is at risk to develop severe toxicity when treated with 5-FU (Etienne et al, 1994; Lu et al, 1995; Chazal et al, 1996).