TREM2 and Alzheimer disease: Given that Aβ40/Aβ42 from APP cleavage in CSF and blood have been reported to correlate with brain pathology (Dey et al., 2019; Wang H. et al., 2020) and that soluble TREM2 in human cerebrospinal fluid has been identified to be a reliable predictor of the early stages of AD (Piccio et al., 2008; Ewers et al., 2019; Suarez-Calvet et al., 2019; Ma et al., 2020; Yang et al., 2020), whether sCSF1R can be determined in human cerebrospinal fluid or the serum of neurodegenerative patients as a biomarker awaits to be explored (Figure 3).