The primary purpose of this study is to prospectively determine whether capecitabine and 5-FU-induced toxicity is preventable by dose reduction prior to start of the first administration in patients heterozygous or homozygous mutant for DPYD*2A, and to determine whether this strategy is cost-effective. Secondly, an individualized treatment algorithm for capecitabine and 5-FU therapy in DPYD*2A mutant patients will be developed and the pharmacokinetic profile of capecitabine and 5-FU will be assessed.