TREM2 and early-onset autosomal dominant Alzheimer disease: Rare variants associated with Alzheimer’s disease (AD) in the Triggering Receptor on Myeloid Cells 2 (TREM2) gene [1, 2], along with common variants in the membrane-spanning 4-domains subfamily A (MS4A) gene cluster, [3–8] have implicated the innate immune system in AD pathogenesis [9].