SLC6A4 and Huntington disease: Gene expression studies have found changes to the expression of Htr1a, Htrb1, and SERT in the brains of R6/1 mice (Pang et al., 2009), suggesting that alterations of mood behavior in HD mice could be caused, at least in part, by a low abundance of these 5-HT receptors and this 5-HT transporter.