A Study to Evaluate the Effect of Venetoclax on Participants Receiving a Covalent Bruton's Tyrosine Kinase Inhibitor (cBTKi) for First-line Chronic Lymphocytic Leukemia (1L CLL) to Achieve Deep Durable Remissions to Allow Off-treatment Period.

Inclusion Criteria:

1. Diagnosed with CLL and currently receiving a stable dose of cBTKi (i.e., ibrutinib,acalabrutinib, or zanubrutinib) for at least 6 months for 1L treatment with aresponse of at least a PR per iwCLL criteria

2. Eastern Cooperative Oncology Group (ECOG) performance score of less than or equal to (</=) 2

3. Adequate renal and liver function

Exclusion Criteria:

1. Prior B-cell lymphoma (Bcl-2) inhibitor therapy

2. Anti-cluster of differentiation 20 (CD20) therapy within the month prior toscreening

3. Progressive or stable disease on cBTKi

4. Transformation of CLL to aggressive non-Hodgkin lymphoma (NHL) (Richter'stransformation or pro-lymphocytic leukemia)

5. History of cardiomyopathy

6. Hypersensitivity to venetoclax or to any of the excipients (e.g., trehalose)

7. Clinically significant cardiovascular disease

8. Active bleeding or history of bleeding diathesis

9. Pregnant women and nursing mothers

10. Uncontrolled autoimmune hemolytic anemia or immune thrombocytopenia