Long somatic DNA-repeat expansion drives neurodegeneration in Huntington's disease.

In Huntington's disease (HD), striatal projection neurons (SPNs) degenerate during midlife; the core biological question involves how the disease-causing DNA repeat (CAG) in the huntingtin (HTT) gene leads to neurodegeneration after decades of biological latency. We developed a single-cell method for measuring this repeat's length alongside genome-wide RNA expression. We found that the HTT CAG repeat expands somatically from 40-45 to 100-500+ CAGs in SPNs.