HTT and juvenile Huntington disease: The HTT gene contains 67 exons, and in the context of an expanded CAG repeat, the HTT mRNA can be alternatively processed, activating polyadenylation (polyA) sites within intron 1 to generate the HTT1a transcript.18,19HTT1a encodes the exon 1 HTT protein (HTTexon1), which is known to be highly aggregation-prone20 and pathogenic.21 As the alternative processing of HTT mRNA, and consequently, the levels of HTT1a and HTTexon1 increase with increasing CAG repeat length,19 HTTexon1 may be the pathogenic effector of somatic CAG repeat expansion in Huntington’s disease.