DPYD and dihydropyrimidine dehydrogenase deficiency: However, due to the considerable number of DPYD mutations and to balance the costs vs. benefits, the analysis of the most frequent variants associated with DPD deficiency, i.e., IVS14+1G>A, c.1679T>G, and c.2846A>T, should be performed before treatment, while the detection of additional rare mutations is recommended in patients suffering from unexpected severe toxicities early after the beginning of treatment, until more affordable technological platforms will allow a cost-effective analysis of all deficient variants associated with the poor-metabolizer status.