APOE and Alzheimer disease: Recently, several heterozygous missense mutations in triggering receptor expressed on myeloid cells 2 (TREM2) have been identified as risk factors for a number of neurodegenerative disorders including Alzheimer’s disease (AD), amyotrophic lateral sclerosis, frontotemporal dementia and Parkinson’s disease.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 For example, the TREM2 R47H variant has been reported to confer an increased risk of AD in comparable odds ratios to APOE4,8, 9 the greatest known genetic risk factor for late-onset AD.13