DRD2 and Huntington disease: Direct support for a causal role for DA and D2-R in HD comes from the recent demonstration that the toxicity of the N-terminal fragments of mHtt is potentiated by dopamine in cells expressing mHtt exon 1 and transgenic HD mouse models (Charvin et al., 2005; Cyr et al., 2006; Stack et al., 2007; Benchoua et al., 2008).