Agrammatic and logopenic PPA are more difficult to distinguish, with the former characterized by breakdown of grammatical production linked to tau protein-based degeneration of the inferior frontal gyrus, and the latter by reduced and slowed speech production, preserved grammar, and repetition deficits, linked to Alzheimer's (beta-amyloid and tau protein) pathology centered in the parietal lobe. This evidence concerns the gene MAPT and primary progressive aphasia.