Entospletinib (GS-9973), an oral, selective inhibitor of spleen tyrosine kinase (SYK), was evaluated as monotherapy in this multicenter, phase 2 study (NCT01799889) of 49 patients with relapsed or refractory chronic lymphocytic leukemia (CLL), including those with Richter's transformation (RT), who had received prior therapy with a B-cell receptor (BCR) inhibitor.