Key Inclusion Criteria:

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          -  B-cell hematologic malignancies expected to express the cluster of differentiation 20
             (CD20) antigen who have relapsed after or failed to respond to at least one prior
             treatment regimen and for whom there is no available therapy expected to improve
             survival

          -  Adequate hepatic, hematologic, and renal function

        Key Exclusion Criteria:

          -  Pregnant or lactating women

          -  Monoclonal antibody, radioimmunoconjugate, antibody-drug conjugate, chemotherapy, or
             other investigational anti-cancer agent within 4 weeks prior to study drug

          -  Treatment with radiotherapy within 2 weeks prior to the first BTCT4465A
             (Mosunetuzumab) administration

          -  Systemic immunosuppressive medication within 2 weeks prior to study drug

          -  Autologous stem cell transplantation (SCT) within 100 days prior to study drug, or any
             prior allogeneic SCT or solid organ transplantation

          -  Autoimmune disease with the exception of controlled/treated hypothyroidism,
             disease-related immune thrombocytopenic purpura, or hemolytic anemia

          -  History of central nervous system (CNS) lymphoma or other CNS disease

          -  Significant cardiovascular or pulmonary disease

          -  Hepatitis B or C or human immunodeficiency virus (HIV)

          -  Receipt of a live attenuated vaccine within 4 weeks prior to study drug

          -  Prior treatment with chimeric antigen receptor T-cell (CAR-T) therapy within 30 days
             before first BTCT4465A (Mosunetuzumab) administration