Genetic polymorphisms in UGT1A1, such as UGT1A1*28 in Caucasians and Asians and UGT1A1*6 only in Asians, contribute to interpatient variability in the pharmacokinetics and toxicities of irinotecan, particularly severe neutropenia (Ando et al, 2000; Innocenti et al, 2004; Minami et al, 2007; Innocenti et al, 2009; Chen et al, 2014). Here, UGT1A1 is linked to neutropenia.