Patients carriers of DPYD c.1905+1G/A or DPYD c.2846A/T and/or UGT1A1*28/*28 genotypes (n = 48) had a higher risk of experiencing grade ≥ 3 overall AEs (OR, 1.89 [95% CI, 1.01–3.53]; P = 0.05), overall hematological AEs (OR, 2.79 [95% CI, 1.51–5.15]; P = 0.001), stomatitis (OR, 3.32 [95% CI, 1.39–7.94]; P = 0.007), neutropenia (OR, 2.98 [95% CI, 1.61–5.52]; P< 0.001) and febrile neutropenia (OR, 2.78 [95% CI, 1.18–6.54]; P = 0.02), compared to patients bearing DPYD c.1905+1G/G and DPYD c.2846A/A and UGT1A1*1/− genotypes (n = 387) (Supplementary Table 6). This evidence concerns the gene UGT1A1 and Decreased total neutrophil count.