PGP9.5 is a member of a gene family whose products hydrolyze small C-terminal adducts of ubiquitin to generate the ubiquitin monomer. Expression of PGP9.5 is highly specific to neurons and to cells of the diffuse neuroendocrine system and their tumors. It is abundantly present in all neurons (accounts for 1-2% of total brain protein), expressed specifically in neurons and testis/ovary. Another potentially protective function of PGP9.5 is its reported ability to stabilize monoubiquitin, an important component of the ubiquitin proteasome system. It is also associated with Alzheimer's disease, and required for normal synaptic and cognitive function.