MAPT and Down syndrome: Furthermore, MAPT is a major candidate involved in the mechanism of 17q21.31 microdeletion syndrome, a pathology characterized by the microdeletion of a small chromosomal region (from 650 to 1,608 kb) containing several genes, including MAPT. The symptoms of 17q21.31 microdeletion syndrome include mental retardation, hypotonia and characteristic facial features.