In a systematic blood-based biomarker review in PD, seven proteins (apolipoprotein A1, apolipoprotein-A IV, inter-alpha-trypsin inhibitor heavy, complement C4A, complement C4B, complement C3, and haptoglobulin) were consistently downregulated in PD compared to the control [87]. The gene discussed is APOA4; the disease is Parkinson disease.