MAPT and Alzheimer disease: In contrast, the 2nd generation tau-PET tracer [18F]PI-2620 binds to PSP-type 4R and AD-type 3/4R tau in vitro [17–19] and shows elevated [18F]PI-2620 PET binding in PSP in subcortical 4R predilection sites such as the pallidum and putamen [15, 20–24].