Three gene-drug pairs were prioritized for initial guideline development: <i>DPYD</i>-fluoropyrimidines, <i>TPMT/NUDT15</i>-thiopurines (paediatric ALL), and <i>CYP2C9/VKORC1</i>-coumarin anticoagulants (e.g., warfarin, acenocoumarol). This evidence concerns the gene CYP2C9 and acute lymphoblastic leukemia.