This clinical trial is for participants with relapsed/refractory follicular lymphoma.

The purpose of this study is to understand more about whether the investigational drug, tazemetostat, in combination with lenalidomide and rituximab will be a useful treatment for people with relapsed/refractory follicular lymphoma. An investigational drug means that it has not yet been approved by the United States Food and Drug Administration (FDA).

Tazemetostat is an oral cancer drug that acts as a EZH2 inhibitor, which blocks activity of the EZH2 gene, which may help keep the cancer cells from growing
   
Prior to starting the study drug, participants will have screening tests completed. Screening tests may be performed over more than one visit, if necessary.

Participants will be randomized into one of two treatment groups. Randomization is like flipping a coin and there is a 50/50 chance of being assigned to either group. The first group will receive tazemetostat plus rituximab and lenalidomide. The second will receive placebo plus rituximab and lenalidomide.

Tazemotostat will be taken twice daily during each 28-day cycle for a total of 12 cycles. The study team will determine the required doses of rituximab and lenalidomide.

After the study drug administration period is completed, a number of the initial screening tests will be repeated.