ARPP21 and Huntington disease: Noticeably, cluster centroid Drd2-1 (Drd2-MSNs: cluster centroid 1, down-regulation across CAG repeats) implicates stress response and cell survival genes such as Hipk4, a kinase that promotes HD pathogenesis in transgenic flies (Al-Ramahi et al., 2018), and Arpp21, a cAMP-regulated phosphoprotein that protects neurons against HD pathogenesis in transgenic Caenorhabditis elegans nematodes (Bigan et al., 2020).