PENK and Huntington disease: Some of the key neuronal genes that are consistently shown to be repressed across human HD brain and in animal models include brain-derived neurotrophic factor (Bdnf), dopamine receptor 2 (Drd2), dopamine receptor 1a (Drd1a), preproenkephalin (Penk1), adenosine A2a Receptor (Adora2a) and protein phosphatase 1 regulatory subunit 1B (Ppp1r1b).