Early Clonal Dynamics During Venetoclax Treatment for Relapsed or Refractory Chronic Lymphocytic Leukemia (CLL)

-INCLUSION CRITERIA: 1. Diagnosis of CLL/SLL which is made according to the updated criteria of the NCI Working Group. 2. Active disease as defined by at least one of the following (iwCLL consensus criteria): * Weight loss \>=10% within the previous 6 months * Extreme fatigue * Fevers of greater than 100.5 degrees F for \>=2 weeks without evidence of infection * Night sweats for more than one month without evidence of infection * Evidence of progressive marrow failure as manifested by the development of, or worsening of * Anemia and/or thrombocytopenia * Massive or progressive splenomegaly * Massive nodes or clusters or progressive lymphadenopathy * Progressive lymphocytosis with an increase of \>50% over a 2-month period, or an anticipated doubling time of less than 6 months 3. Must have designated hematologist/oncologist will assume care and provide venetoclax after the ramp-up phase is complete 4. Must have G6PD testing performed to determine whether rasburicase can be given 5. Must have HLA-testing performed to determine whether allopurinol hypersensitivity exists 6. Age \>=18 years 7. ECOG 0-2 8. Agreement to use acceptable methods of contraception for the duration of venetoclax treatment if sexually active and able to bear or beget children 9. Willing and able to participate in all required evaluations and procedures in this study protocol including swallowing capsules without difficulty 10. Able to comprehend the investigational nature of the protocol and provide informed consent EXCLUSION CRITERIA: 1. Female patients who are currently pregnant or nursing 2. Any uncontrolled active systemic infection 3. Any life-threatening illness, medical condition, or organ system dysfunction that, in the investigator s opinion, could compromise the subject s safety or put the study outcomes at undue risk 4. Known additional malignancy that is progressing or requires active treatment. --Note: Exceptions include basal cell carcinoma of skin, squamous cell carcinoma of skin, and in situ cervical cancer that has undergone potentially curative therapy. Further exceptions include other cancers from which the subject has been diseasefree for \> 2 years, cancers which will not limit survival to \< 2 years or cancers in remission receiving endocrine therapy. 5. Richter s Transformation 6. Any prior therapy with BCL-2 inhibitors 7. Concomitant use of strong CYP3A4 inhibitors 8. Disease significantly affecting gastrointestinal function or absorption 9. Uncontrolled autoimmune hemolytic anemia or autoimmune thrombocytopenia 10. Concomitant systemic cancer directed therapy (e.g. immunotherapy, chemotherapy, radiotherapy) 11. Absolute neutrophil count (ANC) \<1000/microL, platelets (Plt) \<30,000/ microL 12. Serum bilirubin \>3 times upper limit of normal (ULN) 13. Severe psychiatric illness/social situations or cognitive impairment that would limit the patient s ability to tolerate and/or comply with study requirements * If the PI assesses the decreased ANC and/or Plt to be related to CLL involvement, patients may still be enrolled in the study, as cytopenias as expected to improve with treatment of CLL. Patients may receive supportive measures (e.g. transfusions, IVIG, growth factor support, etc.) to avoid severe cytopenias prior to and during therapy with venetoclax.