TREM2 and Alzheimer disease: Efforts are ongoing to develop a Trem2 allele expressing the full-length R47H risk factor at levels similar to wild-type Trem2. APOEε4 is strongly associated with disease development and severity (Bu, 2009; Yamazaki et al., 2019) and at least one allele is present in approximately 65% of AD patients (Mayeux et al., 1998).