Ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1) is a soluble cytoplasmic protein found in neurons and in cells of the diffuse neuroendocrine system, and their respective tumors. Therefore, UCH-L1 has been widely used as a peripheral nerve fiber marker. UCHL-1 functions as a tissue-specific ubiquitin carboxyl terminal hydrolase isoenzyme involved in the processing of both ubiquitin precursors and ubiquitinated proteins.

UCH L1 is down-regulated in brains from Parkinson disease and Alzheimer disease patients, and certian site specific mutations in the UCHL1 gene can either increase or decrese the risk of Parkinson's and/or Alzheimer's neurodegenerative diseases.

Human UCHL 1 and the closely related UCHL3 protein have one of the most complicated knot structures ever discovered, with five knot crossings. This knot structure is expected to help the protein resist degradation in the proteasome.