Inclusion Criteria:

- Patients meeting all the following inclusion criteria will be eligible for
participation in the study:

1. Capable of giving signed informed consent as described in Section 13.2, which
includes compliance with the requirements and restrictions listed in the
Informed Consent Form and this protocol.

2. Aged between 18 and 79 years at the time of signing the Informed Consent Form

3. Ability to comply with the study protocol and procedures and required
hospitalizations, in the investigator's judgement.

4. Eastern Cooperative Oncology Group (ECOG) performance status (PS) of ≤2.

5. Adult patients with previously untreated, histologically proven Richter's
syndrome, diffuse large B cell variants, following WHO 2008 criteria (Swerdlow
SH, 2008).

6. Screening flow cytometry or immunohistochemistry (IHC) evidence of CD20
positive disease as per central review (dim expression of CD20 is acceptable)

7. Adequate BM function independent of growth factor or transfusion at screening
as follows unless cytopenia is clearly due to marrow involvement of CLL:

1. Platelet count ≥75 x 109/L; in cases of thrombocytopenia clearly due to
marrow involvement of CLL (per the discretion of the investigator),
platelet count should be ≥ 30 x 109/L.

2. ANC ≥1 x 109/L unless neutropenia is clearly due to marrow involvement of
CLL (per the discretion of the investigator)

3. Total hemoglobin ≥ 9 g/dL unless anemia is due to marrow involvement of
CLL (per the discretion of the investigator)

8. Measured or estimated creatinine clearance ≥ 45 mL/min by institutional
standard method.

9. Life expectancy > 3 months

10. For women of childbearing potential (WOCBP): agreement to remain abstinent
(refrain from heterosexual intercourse) or use contraceptive methods that
result in a failure rate of < 1% per year, and agreement to refrain from
donating eggs, during the treatment period and for at least 3 months after the
last dose of mosunetuzumab and 3 months after the last dose of tocilizumab (if
applicable).

1. It is recommended to remain abstinent or use contraception for 12 months
after the final dose of cyclophosphamide, doxorubicin, or vincristine.

2. A woman is considered to be of childbearing potential (WOCBP) if she is
postmenarchal, has not reached a postmenopausal state (≥ 12 continuous
months of amenorrhea with no identified cause other than menopause), and
is not permanently infertile due to surgery (i.e., removal of ovaries,
fallopian tubes, and/or uterus) or another cause as determined by the
investigator (e.g., Müllerian agenesis). The definition of childbearing
potential may be adapted for alignment with local guidelines or
regulations.

Examples of contraceptive methods with a failure rate of < 1% per year include bilateral
tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation,
hormone-releasing intrauterine devices, and copper intrauterine devices.

Exclusion Criteria:

1. Pregnant or breastfeeding or intending to become pregnant during the study or within
3 months after the final dose of mosunetuzumab.

a) WOCBP must have a negative serum pregnancy test result within 14 days prior to
initiation of study treatment. If a serum pregnancy test has not been performed
within 14 days prior to receiving first study treatment, a negative urine pregnancy
test result (performed within 7 days prior to study treatment) must be available.

2. Participants who have received any of the following treatments prior to study entry:

a) Treatment with mosunetuzumab or other CD20/CD3-directed bispecific antibodies.

3. Participants who have received any of the following treatments, whether
investigational or approved, given to treat RS, within the respective time periods
prior to initiation of study treatment:

1. Autologous SCT within 100 days prior to first mosunetuzumab administration.

2. Allogeneic stem cell transplant for CLL

3. CAR T-cell therapy for CLL within 100 days before first study treatment

4. Systemic corticosteroid treatment ≤ 20 mg/day prednisone or equivalent to
control symptoms related to disease progression for a maximum of 5 days before
starting C1D1 and inhaled corticosteroids are permitted.

4. Central nervous system (CNS) involvement as documented by spinal fluid cytology or
imaging.

5. Transformation of CLL to prolymphocytic leukemia.

6. History of prior malignancy, except for conditions as listed below if patients have
recovered from the acute side effects incurred as a result of previous therapy:

1. Malignancies treated with curative intent and with no known active disease
present for ≥ 2 years before enrollment.

2. Adequately treated non-melanoma skin cancer or lentigo maligna without evidence
of disease.

3. Adequately treated cervical carcinoma in situ without evidence of disease.

4. Surgically/adequately treated low grade, early stage, localized prostate cancer
without evidence of disease.

7. Any of the following laboratory abnormalities:

1. Calculated creatinine Clearance < 45 mL/min (by institutional standard
method.).

2. Absolute neutrophil count (ANC) < 1.0 x 109/L, unless secondary to bone marrow
involvement by CLL.

3. Platelet count <75 x 109/L except if thrombocytopenia is clearly due to marrow
involvement of CLL (per the discretion of the investigator) for which exclusion
criteria would be platelet count < 30 x 109/L.

4. Serum aspartate aminotransferase (AST)/serum glutamic-oxaloacetictransaminase
(SGOT) or alanine transaminase (ALT)/serum glutamate pyruvate transaminase
(SGPT) >2.5 x upper limit of normal (ULN).

5. Serum total bilirubin > 1.5 x ULN, except in cases of Gilbert's syndrome.

8. History of severe allergic or anaphylactic reactions to humanized or murine
monoclonal antibody therapy (or recombinant antibody-related fusion proteins)

9. Contraindication to tocilizumab.

10. Presence of any autoimmune disorder including autoimmune hemolytic anemia or
autoimmune thrombocytopenia active at the moment of first dose of therapy.

a) Participants with a history of disease-related immune thrombocytopenic purpura or
autoimmune hemolytic anemia may be eligible.

11. History of autoimmune disease, including, but not limited to, myasthenia gravis,
myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis,
inflammatory bowel disease, vascular thrombosis associated with antiphospholipid
syndrome, Wegener granulomatosis, Sjögren syndrome, Guillain-Barré syndrome,
multiple sclerosis, vasculitis, or glomerulonephritis. Exceptions include the
following:

1. Participants with a history of autoimmune-related hypothyroidism on a stable
dose of thyroid replacement hormone may be eligible.

2. Participants with controlled Type 1 diabetes mellitus who are on an insulin
regimen are eligible for the study.

3. Participants with a remote history of, or well-controlled autoimmune disease,
with a treatment-free interval from immunosuppressive therapy for 12 months may
be eligible after review and discussion with the Coordinators.

12. History of solid organ transplantation

13. Participants with infections requiring IV treatment with antibiotics or
hospitalization (Grade 3 or 4) within the last 4 weeks prior to enrollment or known
active bacterial, viral (including SARS-CoV-2), fungal, mycobacterial, parasitic, or
other infection (excluding fungal infections of nail beds) at study enrollment.

14. History of confirmed progressive multifocal leukoencephalopathy (PML)

15. Positive serologic HIV test at screening

16. Positive test results for chronic hepatitis B infection (defined as positive
hepatitis B surface antigen [HBsAg] serology). Participants with occult or prior
hepatitis B infection (defined as positive total hepatitis B core antibody and
negative HBsAg) may be included if hepatitis B virus (HBV) DNA is undetectable at
the time of screening. These participants must be willing to undergo monthly DNA
testing and appropriate prophylactic antiviral therapy as indicated.

17. Acute or chronic hepatitis C virus (HCV) infection. Participants who are positive
for HCV antibody must be negative for HCV by polymerase chain reaction (PCR) to be
eligible for study participation.

18. Known or suspected chronic active Epstein Barr Virus infection (CAEBV).

19. Patients with history of macrophage activation syndrome (MAS)/hemophagocytic
lymphohistiocytosis (HLH)

20. Received a live, attenuated vaccine within 4 weeks before first dose of study
treatment, or in whom it is anticipated that such a live attenuated vaccine will be
required during the study period or within 5 months after the final dose of study
treatment.

21. Left ventricular ejection fraction (LVEF) <50% by multiple-gated acquisition (MUGA)
scan or echocardiogram.

22. Evidence of any significant, concomitant disease that could affect compliance with
the protocol or interpretation of results, including, but not limited to:

1. significant cardiovascular disease (e.g., New York Heart Association Class III
or IV cardiac disease, myocardial infarction within the previous 3 months,
unstable arrhythmia, or unstable angina)

2. significant pulmonary disease (such as obstructive pulmonary disease or history
of bronchospasm)

3. clinically significant history of liver disease, including viral or other
hepatitis, or cirrhosis.

4. current or past history of CNS disease, such as stroke, epilepsy, CNS
vasculitis, or neurodegenerative disease.

5. Participants with a history of stroke who have not experienced a stroke or
transient ischemic attack in the past year and have no residual neurologic
deficits as judged by the investigator are allowed.

6. Participants with a history of epilepsy who have had no seizures in the past 2
years with or without anti-epileptic medications can be eligible.

23. Recent major surgery within 4 weeks prior to first study treatment administration,
with the exception of protocol-mandated procedures (e.g., tumor biopsies and bone
marrow biopsies)

24. Participants who are in dependence to the Sponsor or an investigator.

25. Any serious medical condition or abnormality in clinical laboratory tests that, in
the investigator's judgment, precludes an individual's safe participation in and
completion of the study.