By comparing the plasma level of the 24-OHC concentration and the CYP46A1 rs754203 polymorphism in patients with T2DM with or without MCI, we aimed to clarify whether there is a significant difference in the plasma level of the 24-OHC concentration between the two groups and what role the CYP46A1 rs754203 polymorphism plays in the occurrence of MCI. This evidence concerns the gene CYP46A1 and type 2 diabetes mellitus.