Low power view of a lymph node with CLL/SLL. The darker areas are composed of small, mature B lymphocytes. The paler zones are proliferation centers consisting of larger lymphoid cells with more abundant cytoplasm and dispersed nuclear chromatin. These areas are mitotically active and show a high Ki-67 labeling index. When proliferation centers are large and start becoming confluent (as seen in this image), they should raise concern for transformation to a large cell lymphoma (Richter's syndrome).