If available, a phenotyping test (see main text for further details) should be considered to estimate the starting dose. In absence of phenotyping data, a dose of <25% of the normal starting dose is estimated assuming additive effects of alleles on 5-FU clearance. Therapeutic drug monitoring should be done at the earliest time point possible (e.g., minimum time point in steady state) in order to immediately discontinue therapy if the drug level is too high.