To compare the role of mGluR5 in normal aging and in a neurodegenerative process that progresses with aging, we employed the BACHD mice, which is a mouse model of HD that has a robust and progressive neurodegenerative phenotype27, as well as double mutant mice (BACHD/ mGluR5−/−), which express mutant huntingtin protein in the absence of mGluR5 expression. Here, HTT is linked to Huntington disease.