Although not obvious sex differences in aromatase immunoreactivity have been detected in the brain of AD patients, studies in rodents suggest that aromatase activity may protect the female brain from AD pathology by increasing local estradiol levels but may have a negative impact on the male brain by decreasing local levels of testosterone.129,130 Whether this is the case in the human brain is unknown. This evidence concerns the gene CYP19A1 and Alzheimer disease.