Through the analysis of 443 patients from TRIBE trial, we identified statistically significant associations between DPYD c.1905+1G/A and DPYD c.2846A/T genotypes and grade ≥ 3 hematological AEs and stomatitis, and between UGT1A1*28 variant and grade ≥ 3 hematological AEs, in particular neutropenia, regardless of the treatment arm. Here, UGT1A1 is linked to stomatitis.