HTT and juvenile Huntington disease: Pathogenic huntingtin exon‐1 protein (httex1), characterized by an expanded polyglutamine tract located between the N‐terminal amphiphilic region and a C‐terminal polyproline‐rich domain, forms fibrils that accumulate in neuronal inclusion bodies, and is associated with a fatal, autosomal dominant neurodegenerative condition known as Huntington's disease.