The problem the OP pointed out is a valid one. Alpha-synuclein is a biomarker for many neurological diseases https://pubmed.ncbi.nlm.nih.gov/24262191/ https://

The problem the OP pointed out is a valid one. Alpha-synuclein is a biomarker for many neurological diseases https://pubmed.ncbi.nlm.nih.gov/24262191/ https://pubmed.ncbi.nlm.nih.gov/24262191/ So you might say someone has a marker for Parkinson's and they might get another neurological disorder. They tried these treatments in Alzheimer's and it failed. Why? because it is just another symptom and not the fundamental cause of the disease. It is a good biomarker for poor health, but it is not a good biomarker for any specific disorder.