Genetic studies can offer powerful insights for the development of disease-modifying therapies for Alzheimer’s disease. Protective genetic variants that delay the onset of cognitive impairment have been found in people with sporadic Alzheimer’s disease and in carriers of mutations that usually cause autosomal-dominant Alzheimer’s disease in mid-life. The study of families who carry autosomal dominant mutations provides a unique opportunity to uncover genetic modifiers of disease progression, including rare variants in genes such as APOE and RELN.