SLC18A3 and Alzheimer disease: Although [11C]SA4503 has provided valuable information about S1R distribution in healthy controls, AD, and Parkinson’s disease patients [13], this radiotracer also displays significant affinity for S2R and the vesicular acetylcholine transporters (VAChT) (i.e., S2R Ki = 63 nM [9], VAChT Ki = 50 nM [14]; S2R/S1R = 13.6; VAChT/S1R = 10.9), confounding interpretation of its PET imaging data with respect to S1R.