The breaking point where repeat expansion triggers neuronal collapse in Huntington's disease.

Somatic CAG expansion drives neuronal loss in Huntington's disease (HD), but how expansion results in pathogenesis has remained unclear. Handsaker et al.1 use single-cell RNA and repeat length sequencing to reveal a phased model of expansion and toxicity, highlighting a critical tipping point beyond 150 CAG repeats where neuronal identity collapses and cells die.