CTSD is a lysosomal protease expressed in the brain that has been shown to promote the degradation of mHTT in primary neurons.60 Levels of CTSD in the CSF were reported in one study to be significantly decreased in HD mutation carriers by MS38 and in another to be unchanged between manHD, preHD and controls using PRM-MS.61 Consistent with these reports, we found CTSD to be significantly reduced in the CSF of HD mutation carriers compared with controls (P = 0.044) but not significantly changed across disease stages. This evidence concerns the gene CTSD and Huntington disease.