Irinotecan is widely used in the treatment of colorectal, gastric, and lung 
cancers. However, adverse drug reactions such as severe diarrhea and neutropenia 
limit the dose of this drug. Irinotecan is metabolized by carboxylesterase to 
form an active metabolite, 7-ethyl-10-hydroxycamptothecin(SN-38), which in turn 
is subsequently conjugated by UGT-glucuronosyltransferase 1A1(UGT1A1)to yield an 
inactive form, SN-38 glucuronide(SN-38 G). The UGT1A1 gene polymorphisms 
contribute to the individual variation in adverse events among patients 
administered irinotecan. However, the distribution of polymorphisms shows large 
interethnic differences. The distribution of UGT1A1*28 greatly differs between 
Caucasians and Japanese; the frequency of UGT1A1*28 is high in Caucasians, 
whereas it is low in Asians including Japanese. Recently, it has been 
demonstrated that genetic variants of UGT1A1*6 in addition to UGT1A1*28 are 
associated with the occurrence of adverse events in irinotecan chemotherapy in 
Asians. This review summarizes recent studies to outline the role of UGT1A1*28 
and UGT1A1*6 for irinotecan-induced adverse drug reaction in Japanese cancer 
patients.