In patients who experienced grade III–IV neutropenia, the UGT1A1*6, UGT1A7*3 and UGT1A9*22 polymorphisms showed significant associations only among those who received the weekly regimen (p = 0.015, p = 0.042, and p = 0.024, respectively). This evidence concerns the gene UGT1A1 and Decreased total neutrophil count.