We further show that NfL concentrations in serum are correlated with CSF concentrations, in line with human HD findings12, with the overall level of NfL 9-fold higher in CSF than in serum, affirming the likely CNS origin of the serum NfL, and suggesting that in mice, serum NfL concentrations could be used to estimate CSF levels. This evidence concerns the gene NEFL and Huntington disease.