PECAM1 and Alzheimer disease: We further studied human brain tissues for the possible binding between ApoE and CD31 and found that the ApoE level in microvessels was significantly lower in human AD brains compared with the one in control brains; using double staining and PLA assays, we found that AD brains had lower levels of ApoE‐CD31 binding than the control brains (Figure 7f).