Together, we identified a core peptide of STAs in AD brains, revealed aggregation-relevant phosphorylation sites and translated these findings to develop an accessible CSF biomarker of AD-type tau pathology that will pave the way for the quantification of early-stage soluble (prefibrillar) tau assemblies in CSF and the development of therapies against these soluble pathological entities that may not be detectable using tau-PET. This evidence concerns the gene MAPT and Alzheimer disease.