Of note, as the expression of TREM2 by microglia remains debatable, with evidence suggesting that this marker identifies monocytes rather than microglia (Fahrenhold et al., 2018), with a role for the peripheral TREM2-positive monocytes/macrophages in AD and therefore the role of the systemic immune system in the development of AD cannot be excluded (Rakic et al., 2018). The gene discussed is TREM2; the disease is Alzheimer disease.