The UGT1A1 gene is located on chromosome 2, and it encodes a protein that modifies hepatic bilirubin in order to allow it to be excreted. SNPs that reduce the activity of the UGT1A1 gene therefore tend to increase serum bilirubin levels.
One particular variation in the promoter region of this gene, known as the UGT1A1*28 variant, has attracted the majority of attention, including an FDA approved genetic test applicable to potential users of the drugs irinotecan and tranilast. There are 4 SNPs in dbSNP that describe the same exact variation. They are:
Normally, i.e. in the wild-type UGT1A1*1 allele, there are six (TA) repeats at the position of these variants. The UGT1A1 alleles named based on variations at this positions include:
- UGT1A1*36: five (TA) repeats - leading to increased promoter activity
- UGT1A1*1: six (TA) repeats (this is the normal allele)
- UGT1A1*28: seven (TA) repeats - leading to decreased promoter activity
- UGT1A1*37: eight (TA) repeats - leading to decreased promoter activity
Note that when direct-to-consumer DNA testing companies using DNA chips return genotype results for one of these SNPs in the form of "II", "DI", or "DD", it can be ambiguous as to how many TA repeats the "I" and "D" designations correspond to.