Fig. 2.

Allelic variation at the HTT exon one repeat locus. The schematic diagram shows the typical reference allele structure of the CAG/CCG repeat region in exon one of HTT (top). We here define the number of pure CAGs as Q¹ and the number of additional downstream glutamine-encoding CAA/CAG repeats as Q². Thus, total encoded-glutamine Q^T = Q¹ + Q². We here define the number of copies of the proline codons before the pure CCG repeat as P¹, the number of pure CCGs as P², and the number of copies of the CCT proline codons as P³. Thus, total encoded-proline P^T = P¹ + P² + P³. The figure shows schematic representations of the atypical alleles observed. Repeat codons are depicted: CAG glutamine codons as red boxes; CAA glutamine codons as green boxes; CCG proline codons as blue boxes; CCA proline codons as yellow boxes; and CCT proline codons as pink boxes. Disease-associated alleles with either zero, three or four downstream glutamine codons (Q² = 0, 3 or 4), are indicated with a red upward triangle, green diamond, or green downward triangle, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)