Although our goal is to adjust sNFL measurements to better approximate cNFL, the mathematical strategy to derive such an adjustment is counterintuitive: because we assume that the source of sNFL is CNS (all patients other than HD have CNS disease) and the route of NFL release is via CSF to blood, the sNFL concentration is cNFL modified by confounders. The gene discussed is NEFL; the disease is Huntington disease.