24S-hydroxycholesterol (24(S)-HC), the major cholesterol metabolite in the brain, is critical to brain cholesterol metabolism and turnover. This metabolite is a very potent, direct, and selective positive allosteric modulator of N-methyl-D-aspartate receptors (NMDARs) with a mechanism that does not overlap that of other allosteric modulators. NMDARs are glutamate-gated ion channels that are critical to the regulation of excitatory synaptic function in the CNS. It is known, that concentrations of 24(S)-HC in plasma and cerebrospinal fluid (CSF) are significantly higher in Alzheimer's disease and in mild cognitive impairment patients at early stages of the diseases compared to healthy subjects. It was suggested, that 24-hydroxycholesterol, could be a sensitive biomarker for the evaluation of these two diseases.