APOE and Alzheimer disease: In a 5-year study of 331 dementia-free older adults (47 with MCI), higher plasma Aβ42 levels at baseline predicted incident late-onset depression.137 In another 6-year study of 223 dementia-free individuals, those with depression and lower Aβ42/40 at baseline were at greater risk of incident AD compared to those with non-Aβ depression.138 Of those with Aβ-associated depression, APOE ε4 carriers were at greater risk of AD than non-carriers.