[5][6] UGT-1A is a uridine diphosphate glucuronosyltransferase (UDP-glucuronosyltransferase, UDPGT), an enzyme of the glucuronidation pathway that transforms small lipophilic (fat-soluble) molecules, such as steroids, bilirubin, hormones, and drugs, into water-soluble, excretable metabolites.

The UGT nomenclature committee has compiled a list of these variants, naming each with a * symbol followed by a number.

The UGT1A1*28 has been associated with an increased risk for neutropenia and Diarrhea in patients receiving the chemotherapeutic drug irinotecan[17][18] due to the insufficient excrete the active metabolite SN‐38, which primarily undergoes glucuronidation in livers.

[19][20] The *28 allele has also shown associations with an increased risk for developing diarrhea in patients receiving irinotecan.

Patients who are heterozygous or homozygous for the *6 allele may have a higher risk for developing neutropenia and diarrhea as compared to those with the UGT1A1*1/*1 genotype.