Elevated serum UCB concentrations are also reported in Gunn rats.18 Gunn rats inherit a single point mutation in the coding region of the UGT1A1 gene that truncates and inactivates UGT1A1, leading to complete absence of bilirubin glucuronidation capacity.2, 20 UCB serum concentrations of these animals range between 50 and 200 μmol/L.21 In line with observations in human GS, hyperbilirubinemia beneficially modulated myocardial function and aortic ejection and imparted ischemic stress resistance in Gunn rats.22 Here, UGT1A1 is linked to Hyperbilirubinemia.