Considering insufficient data on the safety of the FX regimen in patients with homozygous UGT1A1*6 or UGT1A1*28, or with heterozygous UGT1A1*6 and UGT1A1*28, as well as the higher rates of neutropenia-related adverse events in our study evaluating mFX, further modification of the irinotecan dose and intensive observation of the clinical course should be considered in patients with homozygous UGT1A1*6 or UGT1A1*28, or heterozygous UGT1A1*6 and UGT1A1*28, especially in Japanese patients. Here, UGT1A1 is linked to neutropenia.