An initial list of protein candidates was prioritized based on existing literature demonstrating altered levels in the CSF of HD mutation carriers, including C1QC,38 C4B,38 CHI3L1 (also known as YKL-40),30,38,39 CLU,40,41 CTSD,38 FAT2,41 NEFL, PDYN,42 PENK,41 and TTR.38,41,43 Additional protein candidates were selected that, to our knowledge, have not been previously measured in HD CSF but were either reported to have altered expression in the striatum of HD patients,44–46 animal models of HD,47,48 or have been implicated in the pathogenesis of HD.49,50. This evidence concerns the gene PENK and Huntington disease.