Gunn rats have almost no UGT1A1 activity compared with ≈30% remaining conjugation activity in human GS patients.2, 8, 12 As a result, Gunn rats usually possess a 50‐fold increase in UCB concentration compared with wild‐type controls, whereas GS patients typically demonstrate 3‐ to 4‐fold increase in UCB concentration compared with persons without GS.2, 8 Another general difference, is that HDL constitutes the greatest lipoprotein fraction in rats, as opposed to humans, whose HDL concentrations typically constitute ≈25% of total circulating cholesterol.2 Here, UGT1A1 is linked to Gerstmann syndrome.