PLP1 and Alzheimer disease: In contrast, the majority of genes which encode proteins of the myelin sheet showed reduced expression levels at AD stages III–IV/0‐C when compared with MA or with AD stages I–II/0‐C: MBP (P = 0.009 when compared with MA), PLP1 (P = 0.003 when compared with AD stages I–II/0‐A), and CNP, MAG, MAL, MOG and MOBP when compared with MA (P = 0.006, P = 0.005, P = 0.006, P = 0.035 and P = 0.041 respectively).