NEFL and supranuclear palsy, progressive, 1: The neurofilament light chain (NfL) has emerged as a marker of ‘non-specific’ axonal injury and neurodegeneration,4 reflecting its predominant expression in axons.5 In people with PSP, NfL levels in plasma and cerebrospinal fluid (CSF) are substantially increased.6,7 Longitudinal assessment of plasma NfL has been implemented in clinical trials in people with PSP8 showing associations with clinical progression.6