The proportion of sarkosyl-insoluble tau was increased in Tg30xtau−/− mice, compared with Tg30 mice. The levels of tau in the sarkosyl-insoluble (A68) and the soluble (Sup2) fractions were analyzed by immunoblotting in the brain and the spinal cord of Tg30 and Tg30xtau−/− mice. The panels show two representative blots for each condition. A–E: Panspecific B19 tau antibody. F–J: Human-specific tau antibody BR21. K–O: Mouse-specific tau antibody mTau5. The B19 and the BR21 antibodies recognize a major 64-kDa insoluble tau species in the brain (A and F) and the spinal cord (C and H) of Tg30 and Tg30xtau−/− mice. Additional minor bands in the sarkosyl-insoluble fraction were detected by the B19 antibody but not by the human-specific BR21 tau antibody. These minor bands are detected by the murine-specific tau antibody (K and M). The proportion of insoluble tau (ratio of insoluble tau to soluble tau) was increased in brain and spinal cord of Tg30xtau−/− mice, as estimated with the B19 antibody (E) and the human-specific BR21 antibody (J). The 110-kDa murine tau isoform (big tau) was detected only in Tg30 mice (D, M, and N). The asterisk in panel N indicates a nonspecific band of mouse immunoglobulin heavy chain detected by the secondary antibody in the soluble fraction of spinal cord (Tg30, n = 5; Tg30xtau−/−, n = 7). *P < 0.05 by unpaired Student's t-test.