MAPT and Alzheimer disease: This disorder has not been as well-studied as the others and unlike the semantic or nonfluent/agrammatic variants which are caused by frontotemporal lobar degeneration pathology (tau or TDP-43), lvPPA appears to be an atypical variant of Alzheimer’s disease (AD), as evidenced by post-mortem studies (Mesulam et al., 2008, Rohrer et al., 2012b), positive amyloid PIB-PET imaging (Leyton et al., 2011, Rabinovici et al., 2008) and a high CSF tau/Aβ ratio (Rohrer et al., 2010a).