• The neuronal marker UCH-L1 is induced in, and specifically augments the oncogeneinduced transformation of, GCB cells.• High levels of UCHL1 identify patients with GC DLBCL with an increased risk for poor outcomes.Gene expression profiling has identified 2 major subclasses of diffuse large B-cell lymphoma (DLBCL). Cases resembling germinal center (GC) B cells (GCB-DLBCL) generally occur in younger patients, have a distinct molecular pathophysiology, and have improved outcomes compared with those similar to activated post-GC cells (activated B-cell DLBCL). We previously found that the ubiquitin hydrolase UCH-L1 is frequently overexpressed in mature B-cell malignancies and is a potent oncogene in mice. The cause for its overexpression in lymphoma, and whether it impacts the outcome of patients with DLBCL is unknown. Here, we show that UCH-L1 reflects GC lineage in lymphoma and is an oncogenic biomarker of aggressive GCB-DLBCL. We find that UCH-L1 is specifically induced in GC B cells in mice and humans, and that its expression correlates highly with the GCB subtype in DLBCL. We also find that UCH-L1 cooperates with BCL6 in a mouse model of GC B-cell lymphoma, but not with the development of multiple myeloma derived from post-GC cells. Despite the typically good outcomes of GCB-DLBCL, increased UCHL1 identifies a subgroup with early relapses independent of MYC expression, suggesting biological diversity in this subset of disease. Consistent with this, forced Uchl1 overexpression had a substantial impact on gene expression in GC B cells including pathways of cell cycle progression, cell death and proliferation, and DNA replication. These data demonstrate a novel role for UCH-L1 outside of the nervous system and suggest its potential use as a biomarker and therapeutic target in DLBCL. (Blood. 2016;127(12):1564-1574