L1CAM and Parkinson disease: We previously developed and extensively validated an assay for the immunocapture of L1CAM‐positive exosomes from serum using polymer‐coated antifouling magnetic beads that reduce non‐specific binding of free α‐synuclein and other abundant serum proteins.8, 17 Using this method we blindly isolated L1CAM‐positive exosomes from 250 μL of serum from patients with PD (n = 60), MSA (n = 36), PSP (n = 81), and CBS (n = 43) and from HC (n = 47).