The present invention provides an in vitro method to aid the diagnosis of Alzheimer's disease (AD) or Mild Cognitive Impairment (MCI), comprising determining the level of expression of one or more tropomyosin isoform(s) corresponding to P09493-3 and/or P09493-1 in a patient's sample. A kit comprising probes that bind to tropomyosin isoforms P09493-3 and P09493-1 is also provided.