In a normal brain, the ratio of 3R/4R tau is 1:1, but it can change across the different pathologies, such as in the case of progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD), which are 4R-dominant, or in the case of Pick’s disease, which is 3R-dominant108,109. This evidence concerns the gene MAPT and red-green color blindness.