Stage 1 is a safety run-in phase, stage 2 is an efficacy and safety phase for an assessment of the EZH2 Mutant Type population and overall FL population regardless of EZH2 mutation status, and optional stage 3 with efficacy and safety phase for subjects with EZH2 mutation. Stage 3 with Mutant Type population alone will be executed in case the efficacy of the overall population in stage 2 fails whilst the efficacy of EZH2 Mutant Type is sufficiently promising. Stage 2 will include 2 futility interim analyses based on ORR for the first futility and PFS for the second one. In addition, there is a possible sample size re-estimation based on PFS. This is to ensure early detection of the presence/absence of clinical efficacy benefit as well as ensuring adequate powering based on the trial results to demonstrate a meaningful efficacy difference.