UGT1A1 and Gilbert syndrome: Ten per cent of people with European or East Asian ancestry and 25% of people with equatorial African descent have at least two functional variants of UGT1A1 associated with the condition, although not all will meet the bilirubin threshold for diagnosing Gilbert’s syndrome.3 36 The high frequency of different UGT1A1 variants causing mild hereditary hyperbilirubinaemia has led to speculation of balancing natural selection whereby physiological benefits of hyperbilirubinaemia are countered by the neurotoxic impact on infants.