GAPDH and Alzheimer disease: However, when using a targeted proteomics approach (total of 16 unique TTBK1 peptides normalized to GAPDH) to quantify total TTBK1 protein levels, we were unable to show an increase in TTBK1 protein expression at earlier stages of the disease (Control n = 10, AD n = 10, Average Braak Stage 3–4; S5 Fig).