RecruitingDrug trialVenetoclax Treatment (26 Cycles) With 6 Cycles or 12 Cycles of Epcoritamab in Patients With Relapsed or Refractory CLL or SLLIn this study, efficacy and safety of 2 regimens that combine the CD3-CD20 T cell engager epcoritamab with venetoclax will be tested in relapsed/refractory CLL and SLL patients. The trial starts with phase I part to establish the recommended dose level (RDL) of epcoritamab in the combination with venetoclax for the phase II trial.