The present invention relates to antibody-based molecules (including single domain antibody fragment, scFv molecules, antibodies, antibody fragments, diabodies, and the epitope-binding domains thereof) that are capable of immunospecifically and selectively binding to the truncated Asp421 epitope of Tau. Such antibody-based molecules are useful to detect pathological Tau protein conformer if present in a biological sample, especially in conjunction with the diagnosis and/or treatment of Alzheimer's disease or other tauopathy, and thus provide a diagnostic for Alzheimer's disease and other Tau pathologies. The antibody-based molecules of the present invention have particular utility as diagnostic markers for, Alzheimer's disease and related tauopathies and as pharmaceutical compositions for the treatment of such conditions.