Figure 4. Simulation of Two Hypothetical Patients with HD.

The variability of repeat-size distribution is zero at birth and increases during the patient's lifetime as a result of the independent stochastic expansion process. At the time of onset (20% of the cells exceed the pathological threshold) a wide distribution in the CAG₄₀ patient with late onset (A) accounts for slower progression (only 35% of cells exceed pathological threshold after 4 y) while a narrow distribution in the CAG₇₀ patient with juvenile onset accounts for faster progression (85% after 4 y).