Neurotrypsin is a central nervous system-expressed serine protease whose truncation or absence causes nonsyndromic mental retardation (1-3). It is most prominently expressed in structures that are involved in the processing and storage of learned behavior and memory, such as the cerebral cortex, the hippocampus, and amygdala (1). Evidence suggests that neurotrypsin has multiple functions, including axonal outgrowth, maintaining neuronal plasticity, and arranging the perineuronal environment, partly in coordination with other proteases including tissue plasminogen activator (3). At least two isoforms of neurotrypsin are known to exist.