MAPT and Alzheimer disease: To clarify these discrepancies, more detailed analyses have looked at the differential accumulation of 3R and 4R tau in the human brain in relation to AD severity (17, 18), to the brain areas investigated (11, 13, 16), and to region-specific differences in susceptibility to NFT formation within the brain (16), as well as in the NFTs of AD patients where three distinct isoform patterns of tau coexist, containing either 3R tau, 4R tau or both.