The levels of SIRT1 were also analyzed in various brain regions of transgenic mice expressing the human apolipoprotein E4 (apoE4) isoform, which is consistently associated with increased risk for AD: SIRT1 levels were significantly reduced in the frontal cortex and its decrease may play a role in apoE4-associated memory impairments [117]. Here, SIRT1 is linked to Alzheimer disease.