PSEN1 and Alzheimer disease: The effect of folic acid supplementation on present Alzheimer’s disease pathology was determined through measurements of serum Aβ (amyloid beta) 40, Aβ 42, Aβ 42/Aβ 40, APP (amyloid precursor protein) mRNA, PS1 (presenilin 1) mRNA, and PS2 mRNA.