MAPT and supranuclear palsy, progressive, 1: When compared with healthy controls, concentrations of CSF total tau (t-tau) and phosphorylated tau (p-tau)181 tend to be normal or low in PSP.41-43 This may be because the predominant tau isoform in PSP, four-repeat (4R) tau, is not released from cells as freely as three-repeat tau and therefore exhibits lower levels in CSF.44 Similar patterns are seen in a number of neurodegenerative disorders, particularly in atypical parkinsonian syndromes (APS).41,42,45 Magdalinou et al.45 found no significant differences in t-tau and p-tau181 levels when comparing PSP with healthy controls and APS.