NRXN1 and early-onset autosomal dominant Alzheimer disease: Fluidic markers of synaptic health are also emerging; among these, growth‐associated protein 43, synaptosomal‐associated protein 25, and synaptotagmin‐1 are the main presynaptic markers that have been applied in Alzheimer disease, followed more recently by proteomic studies that found decreased neurofascin, neuronal pentraxin 1, and neurexin 1 in Alzheimer disease (see Camporesi et al43 for a review).