This study shows that variants in TREM2 are a rare cause of Alzheimer's disease and underscores the role of the microglial cell in the disease mechanism and as a potential target for therapy. Alzheimer's disease is the most common cause of dementia, typically presenting with a progressive loss of cognitive function and memory. It is a complex disorder with a strong genetic component. In the past, genetic studies have identified mutations in three genes — APP (encoding amyloid precursor protein), PSEN1 (encoding presenilin 1), and PSEN2 (encoding presenilin 2) — as the cause of disease in several families, most of whom have early-onset disease. Expansions in C9orf72 are found in families with mixed types of disease. In late-onset disease, the most common form of Alzheimer's disease, the ε4 allele of the apolipoprotein E . . .