MAPT and Alzheimer disease: However, research would suggest that tau, a complex protein in the physiology and progression of AD, should not be considered as a single biomarker categorised solely into T-tau (total tau) and P-tau (phosphorylated tau) and that the many different forms of tau (which we refer to as the “tauosome”) could at least contribute to the complex and multi-target panel required for improved diagnostic interrogation of AD onset and progression.