MAPT and Alzheimer disease: In a study comparing multiple tau proteoforms across oligomeric (O-tau), sarkosyl-insoluble 1 and 2 (SI1 and SI2) and heat stable monomeric (HS-tau) tau fractions from AD brains, pS396/404 (PHF-1 antibody) expression was similar across O-tau, SI1 and SI2 fractions but not detected in HS-tau, again suggestive of pS396 being involved early in tau multimerization [38].