In contrast to PrP in PrD and α-synuclein in PD, the tau molecule in the human brain exhibits 6 isoforms, of which there are 3 isoforms with 4 microtubule-binding repeats (4R tau) and 3 with three repeats (3R tau), resulting from alternative mRNA splicing. The gene discussed is MAPT; the disease is X-linked retinal dysplasia.