• ≥ 18-years-old

• Diagnosis of chronic lymphocytic leukemia or small lymphocytic lymphoma confirmed by the enrolling institution

• Receiving treatment or planning to receive treatment within 30 days with a venetoclax-based regimen as defined below:

‣ Venetoclax monotherapy

⁃ Venetoclax in combination with anti CD20 monoclonal antibody (Patients must have completed the anti CD20 monoclonal antibody portion of the regimen prior to discontinuing therapy)

• Patient has or will have specimen to identify the CLL or SLL clone(s) for future MRD assessments by clonoSEQ® defined as any of the following:

‣ 3-5 bone marrow aspirate slides (banked), 3-5 FFPE slides (banked), banked tumor cells or banked DNA collected from the patient at any time from original diagnosis of CLL or SLL to present

⁃ Peripheral blood, marrow, or lymph node involvement for fresh sample collection

• The screening ID sample (high disease burden sample) can be sent prior to start of venetoclax-based therapy to identify the clone for tracking.

• Patients must have received venetoclax-based therapy for at least 6 months (including dose interruptions)

• If receiving venetoclax in combination with anti-CD20 monoclonal antibody, patients must have completed or discontinued the anti CD20 monoclonal antibody portion of the regimen prior to discontinuing therapy

• Patients must have two MRD-negative assessments (defined by ≥ 10\^-5 sensitivity) on the peripheral blood by the clonoSEQ® assay at least 28 days apart. Given that MRD status is a dynamic endpoint which may improve with time, if a patient is screen failure based on MRD status (i.e. MRD positive at the 10-5 sensitivity), they may be rescreened to assess for study eligibility.

• Patients must have achieved a complete or partial remission with the venetoclax-based regimen. Criteria for remission are per iwCLL 2018 guidelines. For this protocol, bone marrow biopsy/aspirate evaluation is not necessary to determine remission status.