MAPT and Parkinson disease: In Study 4, there were no significant differences in the concentration of ubiquitin between the control, PD, and PSP groups, see Figure 2B. In the PD group, correlations were identified between T‐tau and ubiquitin (ρ = 0.80, p < 0.001) and P‐tau181 and ubiquitin (ρ = 0.93, p < 0.001), but not between Aβ1‐42 and ubiquitin; see Supporting Information Figure S15.