Abstract: The present application relates to a digested DDIT4L product as a diagnostic marker for Alzheimer's disease, and use thereof in diagnosing Alzheimer's disease. In particular, the present application relates to use of a substance for detecting a digested intron retention (DIR) product encoding DNA-damage-inducible transcript 4 like (DDIT4L) in a sample of a subject in preparing a product for diagnosing Alzheimer's disease or a mild cognitive disorder and/or assessing (e.g., grading or staging) cognitive disorder progression, a related product thereof, and a method for screening a medicament using the DIR product.