MAPT and Alzheimer disease: Similarly, P-T231 was systematically associated with P-S237 (and sometimes P-S238) in AD soluble tau, but P-T231 was observed alone in tau aggregates; and P-S396 was systematically associated with P-S400 (and sometimes P-S404), but P-S396 was observed alone in tau aggregates, suggesting a protective effect of these additional sites of phosphorylation against tau aggregation.