MAPT and Alzheimer disease: AD accounts for the majority of tauopathy cases and is marked by predominantly PHF of tau with an approximately equal 3R to 4R ratio in the microtubule-binding domain, mixed with straight filaments.7, –9 In contrast, PSP and CBD tau aggregation forms straight filaments of tau, which are primarily composed of the 4R isoform.5,7,10, , –13