In the present study, we have confirmed our previous finding of measurable levels of the MAP kinase ERK1/2 in human CSF [8], and we have applied the methods established in that previous work to evaluate the CSF-ERK1/2 concentrations in an age-matched sample including AD patients, MCI patients who converted to AD during followup (MCI-AD), MCI patients who remained stable, and a nondemented reference cohort. The gene discussed is MAPK3; the disease is Alzheimer disease.