UCHL1 and early-onset autosomal dominant Alzheimer disease: Ubiquitin C‐terminal hydrolase L1 (UCH‐L1) is a neuron‐specific enzyme that is highly abundant in the brain.6 UCH‐L1 is a deubiquitinating enzyme that is required for normal cognitive function.7, 8 UCH‐L1 has been implicated in the pathophysiology of Parkinson's disease (PD),9 Alzheimer's disease (AD),10 Huntington's disease,11 and epileptic seizures.12 Increasingly, UCH‐L1 is recognized as a biomarker of brain injury.