RTN4 and Alzheimer disease: Interestingly, comparing the tested proteins concentrations between all investigated groups, only RTN-4 levels differed significantly between almost all study groups (AD vs. PD p < 0.001, W = −6.915; AD vs. MS p < 0.001, W = −7.195) excluding comparison between PD and MS (p = 0.929, W = −0.862) (Figure 2).