We identify a robust subset of potential reference proteins (“cluster 11”) from which we further characterize six specific reference protein candidates (NTRK3, NTRK2, BLMH, CBLN4, PTPRN2 and PTPRS) that can significantly improve the accuracy of key AD biomarkers (Figs. 3, 4). The gene discussed is PTPRS; the disease is Alzheimer disease.