NRGN and Alzheimer disease: In vivo biomarker studies have demonstrated that synaptic degeneration, as indicated by cerebrospinal fluid (CSF) proteins such as growth-associated protein-43, synaptosomal-associated protein 25 (SNAP25) and neurogranin (Ng), precedes abnormalities in established axonal/neuronal degeneration biomarkers (such as hippocampal volume (HCV) and neurofilament light chain (NfL))5, supporting the importance of evaluating both synaptic and neuronal degeneration separately to better model AD progression.