MHCI complex expression increases with aging across the CNS. The invariant chain of MHCI (Beta-2 microglobulin, β2M) as well as the classical MHCI isoforms (H2-D1 and H2-K1) increase in expression with age across the CNS regions examined. While β2M did not show any sex effects, H2-D1 and H2-K1 demonstrate sex and interaction effects in a number of the brain regions examined. Y=Young, A=Adult, O=Old/Aged. Two-way analysis of variance (Age × Sex), **p < .01, ***p < .001 Age effect, ##p < .01, ###p < .001 Sex effect, ^p < .05, ^^^p < .001 interaction, Benjamini–Hochberg Multiple Testing Correction, n.s. not significant, n = 7–8/group.