HTT and Huntington disease: Additionally, evidence from several HD transgenic models demonstrates mutant htt expression selectively downregulates EAAT2 mRNA and protein levels that ultimately results in extracellular glutamate accumulation, excitotoxicity, and neuronal cell death (Behrens et al., 2002; Liévens et al., 2005; Shin et al., 2005; Bradford et al., 2009; Meunier et al., 2016).