A Study to Assess Efficacy and Safety of Oral Tazemetostat in Adult Participants With Relapsed/Refractory Follicular Lymphoma That Does Not Have an "EZH2 Gain-of-function" Genetic Mutation

The purpose of this research study is to learn about the effectiveness and safety of the study drug, tazemetostat, in adults with relapsed/refractory follicular lymphoma whose tumours do not have an "EZH2 gain-of-function" genetic mutation. Follicular lymphoma is a blood cancer. It affects white blood cells called lymphocytes. White blood cells normally help to fight infections, but when you have follicular lymphoma, the blood cells can form tumours in your body. 'Relapsed/refractory' follicular lymphoma means the disease has either not improved or is getting worse (progressing) during or after previous treatment. Tazemetostat already has approval in the United States for the treatment of adult patients with relapsed/refractory follicular lymphoma with or without the "EZH2" mutation who have no satisfactory alternative treatment options. This study is being conducted to better understand the effectiveness in patients whose tumours do not have an "EZH2 gain-of-function" genetic mutation and who previously received therapies commonly used in the U.S. in your body. 'Relapsed/refractory' follicular lymphoma means the disease has either not improved or is getting worse (progressing) during or after previous treatment. Tazemetostat already has approval in the United States for the treatment of adult patients with relapsed/refractory follicular lymphoma with or without the "EZH2 gain-of-function" mutation who have no satisfactory alternative treatment options. This study is being conducted to better understand the effectiveness in patients whose tumours do not have an "EZH2" genetic mutation and who previously received therapies commonly used in the U.S. In this study, all participants will receive the study drug. It will be taken by mouth (orally), as a tablet, twice daily. The sizes and number of tumours according to scan results will be collected as well as results of safety tests (such as physical examinations and laboratory tests). The study consists of 4 periods: * Screening period may take up to 4 weeks and require at least 1 visit. * Treatment period will require 2 visits for each of the first 2 months, followed by 1 visit every month for the remainder of the first 12 months, followed by 1 visit every 3 months (except for women of childbearing potential \[WOCBP\], who will continue to have a pregnancy testing every month) until unacceptable toxicity, disease progression, or the start of new systemic anticancer therapy, whichever is first. * Safety follow-up period will last for 1 month after the last dose of tazemetostat, and it will end with 1 visit or telephone call. * Long-term follow-up period is only for participants who stop taking tazemetostat while their disease continues to respond; this period will last until disease progression, start of new cancer treatment, or death from any cause, whichever is first, and will require a visit every 3 months. Tazemetostat will be provided to participants who tolerate it for as long as their disease does not progress. Participants may be transferred to another study or program after about 2 years for continued treatment with tazemetostat or for long-term follow-up. Patients may withdraw consent to participate at any time.