We demonstrate the accumulation of somatic expansion of the HTT CAG repeat in blood DNA over time in HD-ISS stages 0 and 1 and, critically, show that it is associated with both brain atrophy and CSF NfL, a marker of neuronal–axonal injury, and CSF PENK, a surrogate marker of striatal MSN state. The gene discussed is NEFL; the disease is Brain atrophy.