Genes other than metabolic ones can be responsible for 5FU toxicity, e.g., drug transporters, e.g., by ABC1 may be important in activity and resistance, evidenced by enhanced ABC5 expression in colorectal cancer patients after 5‐FU‐based chemotherapy. ABCB11 is involved in efflux transport of FdUMP (reviewed in 15). This evidence concerns the gene ABCB11 and colorectal cancer.