LDN-57444 is a cell permeable ubiquitin C-terminal hydrolase (UCH-L1) inhibitor (Ki=0.4 μM). Decreases proteasome activity and increases levels of ubiquitinated proteins. Induces apoptosis and causes dramatic alterations in synaptic protein distribution and spine morphology in vivo. LDN-57444 is a useful tool to study UCH-L1 in different pathological conditions such as cancer and neurodegeneration.