The basic model of the HD locus encompasses a divergently transcribed pair of homeodomain1 (Hd1) and homeodomain2 (Hd2) genes that form a functional group, flanked by the mitochondrial intermediate peptidase (MIP) gene on one side, and the β-flanking gene on the other (James et al. 2004a; Kües et al. 2011). The gene discussed is HDAC2; the disease is Huntington disease.