The abundance of FL-APP and CTF is not altered by trisomy of Hsa21. A–D, The relative abundance of FL-APP, APP β-C-terminal fragment (β-CTF), and APP α-C-terminal fragment (α-CTF) compared with β-actin was measured by western blot using A8717 primary antibody in the cortex at 3 months of age in female and male mice. A, Significantly less FL-APP was observed in mice in which App was humanized and mutated (F(1,19) = 23.837, p < 0.001). An additional copy of Hsa21 did not alter FL-APP abundance (F(1,19) = 0.599, p = 0.449). B, Significantly less CTF-α was observed in mice in which App was humanized and mutated (F(1,19) = 5.950, p = 0.025) but an additional copy of Hsa21 did not alter α-CTF abundance (F(1,19) = 3.012, p = 0.099). C, Significantly more β-CTF was observed in mice in which App was humanized and mutated (F(1,19) = 868.431, p < 0.001). By ANOVA, a significant effect of Hsa21 on CTF-β abundance was detected (F(1,19) = 23.462, p < 0.001); however, WT and Tc1 (Bonferroni pairwise comparison p = 1.000) and AppNL-F/NL-F and Tc1;AppNL-F/NL-F (Bonferroni pairwise comparison p = 0.118) were not statistically significant. WT female n = 4, male n = 4: Tc1 female n = 3, male n = 5; AppNL-F/NL-F female n = 2, male n = 2; Tc1;AppNL-F/NL-F female n = 4, male n = 4. D, Representative image of western blot in WT, Tc1, AppNL-F/NL-F, and Tc1;AppNL-F/NL-F mice. Error bars indicate SEM, *p < 0.05 and ***p < 0.001. Data points represent independent mice.