We established that α‐synuclein level in L1CAM‐immunocaptured exosomes above 14 pg/mL is a robust biomarker across cohorts that distinguishes Parkinson's disease from MSA (AUC, 0.90 vs 0.98) or 4‐repeat tauopathies (AUC, 0.93 vs 0.94). The gene discussed is L1CAM; the disease is multiple system atrophy.