lt brain compared to 20% and 100% for newborn and AD brain respectively, indicating that adult tau protein is not extensively phosphorylated. Furthermore, developmental studies in rodents have shown that while some epitopes, such as pT205 or pT181, are highly phosphorylated in the post-embryonic brain, their signal is very faint in adult brain, which could explain why AT8 and AT270 are more prone to display non-specific Igs signal. On the other hand, epitopes such as pS202 or pS396 and pS404 are abundant in the adult brain, and do not display overt non-specific signal in our experiments. We were surprised to see a non-specific signal around 37 kD with the MC1 antibody in Tau KO mice. This antibody recognizes an abnormal conformation of tau encompassing amino acids 5–15 and 312–322. We think that this non-specific signal is due to the knock-in of the EGFP coding sequence into the first exon disrupting the expression of the Mapt gene, and resulting in a chimeric protein with EGFP fused to the first 31 amino acids of tau. Indeed, the same band was detected with an anti-GFP antibody and disappeared in the HS fraction. These results indicate that, somehow, the fusion of EGFP to the small sequence of tau was able to mimic the MC1 epitope. To help improve the detection of tau signal, we first used secondary antibodies designed to bind native Igs. Because Igs coming from the samples are denatured, these antibodies recognize only the primary antibodies, and eliminate the interference of Igs heavy and light chains during the Western blot procedure. Indeed, TrueBlot antibodies completely removed the non-specific signal from problematic primary antibodies in TKO, WT and 3xTg mice, allowing for the visualization of tau signal without interference. The use of TrueBlot antibodies did not necessitate an important modification of the standard Western blot procedure as they replaced conventional secondary antibodies. However, we observed that it was sometimes difficult to obtain a signal with TrueBlot secondary antibodies; hence, we had to increase the amount of protein being loaded or alter their incubation time from 1 h at room temperature for standard secondary antibodies to overnight or more at 4uC with TrueBlot. As an alternative to TrueBlot, we also tested secondary antibodies designed to bind the light chain of Igs at 25 kDa and do not recognize the heavy chain at 50 kDa. These antibodies recognize only the primary antibodies and the light chain of Igs on the membrane, and eliminate the interference of Igs heavy chains in Western blot procedure. Indeed, anti-LC antibodies completely removed the non-specific signal from problematic primary antibodies in TKO, WT and 3xTg mice, allowing a visualization of tau signal without interference. Furthermore, the use of LC antibodies has several advantages versus TrueBlot antibodies: they are less expensive, can be diluted more, and incubated at room temperature for 1 h.