In FFPE-IHC, all antibodies labelled Tau in either mouse or human brain sections, but we found that: (i) the 77E9 antibody labelled Tau only in human brain sections and only following dephosphorylation with λPP; while (ii) the RD3 and RD4 antibodies against 3R and 4R Tau isoforms, respectively, labelled physiological Tau in mouse brain sections, but failed to label Tau in human tauopathy brain sections. This evidence concerns the gene RD3 and tauopathy.