Primary CNS lymphoma, diffuse large B-cell type, is an aggressive tumor with a high Ki-67 labeling index (usually >20%; approaching almost 90% in this case). The percentage of Ki-67 expressing cells reflects the proportion of the tumor cells that are actively cycling. The prognostic significance of Ki-67 expression in diffuse large B-cell lymphoma is controversial. Image courtesy of: Dr. Luciano de Souza Queiroz, Dept. of Pathology, Faculty of Medical Sciences, State University of Campinas (UNICAMP), Campinas, São Paulo State, BRAZIL.