• Able to provide written informed consent form approved by the National Ethics Committee (NEC) prior to the initiation of any screening or study-specific procedures and able to understand and to comply with the requirements of the study and the schedule of assessments.

• Histologically documented diagnosis of cFL (CD20+ by flow cytometry or immunohistochemistry) as defined in the International Consensus Classification of Mature Lymphoid Neoplasms (Campo E., 2022) and in the World Health Organization Classification (WHO) 5th edition 2022.

• Age ≥18 years.

• Relapsed or refractory disease. Histologic confirmation of FL relapse is not mandatory but is highly recommended.

• At least one and up to three lines of systemic therapy containing an anti-CD20 antibody (anti-CD20 alone and/or in combination with radiotherapy is not considered as a line of therapy).

• FL requiring systemic therapy assessed by investigator based on tumor size and/or Groupe d'Etude des Lymphomes Folliculaires criteria.

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

• Availability of histological material for centralized revision. Central pathology review is not mandatory for start of treatment.

• At least one measurable or evaluable site of disease at relapse as documented by CT scan (nodes ≥ 1.5 cm in the longest transverse diameter) or FDG-PET (avid FDG sites). Note: MRI is allowed only if CT scan cannot be performed. Patients with exclusive bone marrow involvement are eligible.

⁃ Adequate hematological counts defined as follows:

∙ Absolute neutrophil count (ANC) \> 1.0 x 109/L;

‣ Platelet count ≥ 75 x 109/L;

‣ Hemoglobin ≥ 9 g/dL.

⁃ Adequate renal function defined as creatinine clearance ≥ 40 mL/min (Cockcroft-Gault formula, normalized to 1.72 m2).

⁃ Adequate hepatic function per local laboratory reference range as follows:

∙ Aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 2.5 x ULN;

‣ Bilirubin ≤1.5 x ULN (unless bilirubin rise is due to Gilbert's syndrome or of non-hepatic origin).

⁃ Subject must be able to adhere to the study visit schedule and other protocol requirements.

⁃ Subject must be able to swallow capsules or tablets.

⁃ Women must be:

∙ postmenopausal for at least 1 year (must not have had a natural menses for at least 12 months);

‣ surgically sterile (have had a hysterectomy or bilateral oophorectomy, tubal ligation, or otherwise be incapable of pregnancy);

‣ if they are childbearing potential, completely abstinent (periodic abstinence is not permitted) or if sexually active, be practicing a highly effective method of birth control (e.g., prescription oral contraceptives, contraceptive injections, contraceptive patch, intrauterine device, double barrier method (e.g.: condoms, diaphragm, or cervical cap, with spermicidal foam, cream, or gel, male partner sterilization), before entry, and must agree to continue to use the same method of contraception throughout the study and for 30 days after receiving the last dose of zanubrutinib and 3 months after receiving the last dose of mosunetuzumab.

⁃ Women of childbearing potential must have a negative pregnancy test at screening.

⁃ Men must agree to use an acceptable method of contraception for the duration of the study and for 1 week after receiving the last dose of study drug.

⁃ Male even if surgically sterilized (i.e., status post vasectomy) must agree to 1 of the following:

∙ practice effective barrier contraception (e.g.: condoms) , or

‣ agree to practice abstinence, when this is in line with the usual lifestyle of the subject (periodic abstinence is not permitted).