Dentatorubral-Pallidoluysian Atrophy (DRPLA, AD)

A 9-year-old boy of Japanese descent presents with intractable generalized tonic-clonic seizures, myoclonus, progressive intellectual decline, and cerebellar ataxia since age 6. His father developed chorea, dementia, and gait ataxia at age 38 and is now wheelchair-bound. MRI shows atrophy of the cerebellum, brainstem, and cerebral white matter. Genetic testing reveals a heterozygous CAG repeat expansion (75 repeats) in the ATN1 gene on the structure marked **A** in the diagram. Which of the following best describes the chromosomal location and pathogenic threshold for this mutation?