MAPT and Alzheimer disease: One study found no change in CSF YKL-40 levels in individuals with reduced levels of Aβ42but no cognitive or psychiatric symptoms, indicating that they were in the preclinical stage of AD, while high YKL-40 levels were found in a patient group with abnormal AD biomarkers (Aβ42, t-tau, and p-tau) and subtle memory deficits, classified as prodromal AD [61].