TREM2 and Alzheimer disease: The Song et al. generated TREM2 common variant (TREM2CV) and TREM2 R47H variant (TREM2R47H) Tg mouse lines backcrossing with mouse TREM2−/− to obtain normal TREM2 levels and then crossed with 5xFAD background to study functions of TREM2 R47H variant in the presence of human AD pathology [132].