In addition, future studies should address the questions (i) if CSF-NFL could also aid to differentiate potential PPMS disease mimics, e.g., hereditary spastic paraplegia, and (ii) if composite measurement of DTI CST involvement, viewed together with CSF-NFL levels, may provide a better correlate with the overall functional state of PPMS patients. The gene discussed is NEFL; the disease is hereditary spastic paraplegia.