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Neurofilament light chain (NFL or NEFL) is a subunit of neurofilament protein (NF), which is a class IV intermediate filament that exists specifically in neurons. NFL constitutes NF together with neurofilament heavy chain (NFH), neurofilament medium chain (NFM), α-internexin and peripherin. NF can be expressed in the cell body, dendrites and axons of neurons, and the highest expression level is in the axons. It is the main cytoskeleton protein of neuron axons, and plays a role in maintaining the stability of axons and ensuring the transmission of neuron signaling functions.

Under normal physiological conditions, axons can slowly release a small amount of NFL. However, under various pathological conditions, the release of NFL increases significantly and can freely exchange in interstitial fluid and cerebrospinal fluid. NFL in the cerebrospinal fluid (CSF) can further spread to the blood, where the NFL is highly correlated with the concentration of NFL in CSF, but the concentration is lower than that of NFL in CSF. Concentrations of NFL CSF and blood can indirectly reflects the level of neuropathy, so NFL is clinically used as a biomarker of neuron injury-related diseases.