KLK6 and Alzheimer disease: The conflicting results of either positive or negative associations between CSF KLK6 levels and Aβ42 levels between cohort 1 and cohort 2 further suggest a potential involvement of KLK6 in AD pathological processes at different stages of disease progression, with patients with AD in cohort 1 being both older and more advanced in their cognitive dysfunction than the patients with AD in cohort 2.