A Study to Assess the Efficacy, Safety, Pharmacodynamics, and Pharmacokinetics of Tazemetostat in Combination With Lenalidomide Plus Rituximab Versus Placebo in Combination With Lenalidomide Plus Rituximab in Adult Patients at Least 18 Years of Age With Relapsed/Refractory Follicular Lymphoma.

The participants of this study would have relapsed/refractory follicular lymphoma. Follicular lymphoma is a type of blood cancer. It is referred to as 'relapsed' when the disease has come back after a period of improvement after that follows a treatment regimen and 'refractory' when treatment no longer works. Stage 1 of this trial studied the safety and the level that adverse effects of each of the study drug combinations can be tolerated (known as tolerability). It is also designed to establish a recommended study drug dosage for stage 2 and 3. Stage 1 of the study is completed. Stages 2 and 3 were designed to evaluate and compare how long participants live without their disease getting worse when receiving the study drug in combination with other drug treatment versus the placebo (dummy drug) in combination with other drug treatment. However, following an urgent safety measure, treatment with tazemetostat and placebo was discontinued, enrollment was stopped, and the study was unblinded. As a result, post-urgent safety measure analyses are descriptive in nature.

Inclusion Criteria: 1. Have voluntarily agreed to provide written informed consent and demonstrated willingness and ability to comply with all aspects of the protocol. 2. Males or females are ≥18 years of age, or per country adult legal age regulations, at the time of providing voluntary written informed consent. 3. Life expectancy ≥3 months before enrollment. 4. Meet requirement for hepatitis and human immunodeficiency virus (HIV) infection as follows * Negative serologic or polymerase chain reaction (PCR) test results for acute or chronic hepatitis B virus (HBV) infection Note: Participants whose HBV infection status could not be determined by serologic test results have to be negative for HBV-DNA by PCR to be eligible for study participation. Participants seropositive for HBV with undetectable HBV DNA by PCR are permitted with appropriate antiviral prophylaxis. * Negative test results for hepatitis C virus (HCV) Note: Participants who are positive for HCV antibody must be negative for HCV RNA by PCR to be eligible for study participation * If HIV positive, HIV infection is controlled. Based on Cancer Clinical Trial Eligibility Criteria: Patients with HIV, Hepatitis B Virus, or Hepatitis C Virus Infections - Guidance for Industry (https://www.fda.gov/media/121319/download), patients with HIV should be considered eligible if they have CD4+ T-cell counts ≥ 350 cells/uL and in general, if they have not had an opportunistic infection within the past 12 months. Other exclusion criteria should be considered regarding the drug-drug interaction if antiviral drugs are used. Therefore, in case of controlled HIV infection, since antiviral drugs are used, trial patients should be on established ART for at least 4 weeks and have an HIV viral load less than 400 copies/mL prior to enrolment. 5. Have histologically confirmed FL, Grades 1 to 3A. 6. Must have been previously treated with at least 1 prior systemic chemotherapy, immunotherapy, or chemoimmunotherapy: a. Systemic therapy includes treatments such as: i. Rituximab monotherapy ii. Chemotherapy given with or without rituximab iii. Radioimmunoconjugates such as 90Y-ibritumomab tiuxetan and 131I-tositumomab. b. Systemic therapy does not include, for example: i. Local involved field radiotherapy for limited-stage disease ii. Helicobacter pylori eradication c. Prior investigational therapies will be allowed provided the subject has received at least 1 prior systemic therapy as discussed in Inclusion Criterion #6a. d. Prior autologous/allogeneic hematopoietic stem cell transplant (HSCT) will be allowed. e. Prior chimeric antigen receptor T-cell therapy (CAR T) will be allowed. 7. Must have documented relapsed, refractory, or PD after treatment with systemic therapy (refractory defined as less than PR or disease progression \