HTT and juvenile Huntington disease: In the presence of an expanded CAG repeat, aberrant processing of the huntingtin mRNA generates the HTT1a transcript, which encodes the aggregation-prone and pathogenic exon 1 HTT protein.7 This occurs in post-mortem brains and fibroblast cultures from Huntington’s disease patients8 and has been shown to correlate with disease progression and HTT aggregate deposition in knock-in mouse models.29 Here we demonstrated that human HTT1a is produced throughout the brain in YAC128 mice, that are transgenic for the entire human HTT gene.