APOE and Alzheimer disease: In addition, secreted PDGFRβ (sPDGFRβ), a biomarker of PC injury, is elevated in the cerebrospinal fluid (CSF) of APOE4 carriers (Ishii and Iadecola, 2020; Montagne et al., 2020) and has been shown to be independent of pathological changes in AD (Montagne et al., 2020).