DRD1 and Huntington disease: Finally, although protein phosphatase one regulatory inhibitor subunit 1A Ppp1r1a (down-regulation assigned to Drd1-MSNs: pathogenic response that is mitigated) is not formally linked to neurobehavioral phenotypes in HD models, protein phosphatase inhibition may be relevant to several diseases, including NDs (Vintonyak et al., 2011).