The hepatic isoform 1A1 of uridine diphosphate glucuronosyltransferase is 
responsible for glucuronidation and detoxification of SN-38, the active 
metabolite of irinotecan. The presence of an additional TA repeat in the TATA 
sequence of the UGT1A1 promoter leads to a significant decrease in SN-38 
glucuronidation. Patients with the UGT1A1 (TA)7 allele are more likely to 
experience severe neutropenia and diarrhea following irinotecan chemotherapy. We 
assessed the distribution of the UGT1A1 (TA)n polymorphism in healthy male and 
female US residents of European and Asian descent. We used a fluorescent 
polymerase chain reaction-based assay to detect UGT1A1 (TA)n polymorphisms in 
138 healthy volunteers (56 Caucasians, 37 Chinese, 37 Filipino and eight 
Japanese) between the ages of 18 and 65 years. The chi-test was used to assess 
between-group differences in the distribution of UGT1A1 (TA)n genotypes. The 
UGT1A1 (TA)6/6 genotype was significantly more common in Asians than in 
Caucasians (76 vs. 46%), whereas the (TA)6/7 (39 vs. 20%) and (TA)7/7 (13 vs. 
5%) genotypes were more common in Caucasians than in Asians. Genotype 
distributions did not differ significantly between men and women in either 
group. The UGT1A1 (TA)5/5 genotype was detected in one Caucasian woman. In 
conclusion, consistent with previous reports, the UGT1A1 (TA)7/7 genotype was 
significantly more common in Caucasians than in Asians. UGT1A1 (TA)n/n genotype 
distribution did not vary with sex in individuals of European or Asian descent.