However, the levels/expression of the receptor, unlike transgenic mice, are usually unaltered in AD brains [17], [18], but there is evidence that the receptor levels/expression can be decreased selectively in the hippocampus of AD patients carrying two copies of APOE ε4 allele [17] or increased in the cortical region of familial cases with a PSEN1 mutation [18]. Here, PSEN1 is linked to Alzheimer disease.