NEFL and Brain atrophy: Similarly, neurofilament light (NfL)—a key component of the neuronal cytoskeleton and marker of axonal degeneration—is elevated in HD gene carriers,15 predicting both brain atrophy and clinical and cognitive decline.16mHTT (mutant HTT), the pathogenic agent in HD, and its concentration may be interpreted as a direct measure of the genetic–pathological burden in HD that appears to be neuronal in origin.17 These 2 proteins can be reliably quantified in cerebrospinal fluid (CSF)18, 19 and are among the earliest detectable changes in HD.20