UGT1A1 and Gerstmann syndrome: GS is characterized by reduction of UGT1A1 activity levels to approximately 25% to 30% as a result of homozygous, compound heterozygous, or heterozygous mutations in UGT1A1. [6] The genetic basis of GS was first identified in 1995 after the discovery of the A(TA)7TAA mutation, where an additional TA is inserted in the TATA box of the UGT1A1 proximal promoter.[16]