Neurofilament Light (or NfL) is a cytoskeletal protein that is highly specific for neurons. While NfL is normally present is low levels in body fluids due to slow turnover in neurons, it is released from neurons due to injury or disease, thereby resulting in increased levels in the CSF and/or blood. NfL can be measured in fluids, such as plasma, serum, and CSF by sensitive assays, such as Simoa.