Background: Tazemetostat is a potent and selective inhibitor of the histone methyltransferase enhancer of zeste-homolog 2 (EZH2), the catalytic subunit of the polycomb repressive complex 2 (PRC2). EZH2 is responsible for trimethylation of lysine 27 on histone H3, which results in chromatin remodelling and repressed transcription. Aberrant EZH2 activity has been implicated as an oncogenic driver in non-Hodgkin lymphoma and other malignancies. The SWI/SNF complex also remodels chromatin, activates transcription, and acts in opposition to PRC2. Oncogenesis from mutation and/or loss of the SWI/SNF subunit INI1 in cancers such as malignant rhabdoid tumor (MRT) or SMARCA4 loss in ovarian “small cell carcinoma, hypercalcemic type” is associated with anticancer activity of EZH2 inhibition in preclinical models. This has translated into objective responses of promising durability in subjects (subjs) whose solid tumors exhibited either INI1 or SMARCA4 loss in the tazemetostat phase I adult study. Methods: This phase I, multi-center dose escalation (DEsc) and dose expansion (DExp) study administers tazemetostat twice daily (BID) using an oral suspension formulation in pediatric subjs age 6 months to 21 years. Approximately 24 subjs will be dosed using a “Rolling 6” DEsc design at a starting dose of 240 mg/m² BID. Following DEsc, a DExp cohort will enroll up to 20 subjs. Eligible malignancies includerhabdoid tumors (MRT, atypical teratoid rhabdoid tumor, rhabdoid tumor of the kidney, MRT of ovary) that are INI1 or SMARCA4 negative, synovial sarcoma, and other INI1 negative tumors including epithelioid sarcoma, renal medullary carcinoma, myoepithelial carcinoma, and others. Response assessments are evaluated every 8 weeks using RANO or RECIST 1.1. Primary endpoints include dose-limiting toxicities/maximum tolerated dose (DEsc) and overall response rate (ORR) (DExp). Secondary endpoints include ORR (DEsc), progression-free survival, overall survival, safety/tolerability, PK and duration of response for confirmed responders. The first subj was dosed in January 2016 in the US with additional sites to be opened in Canada, the EU and Australia (NCT02601937). Clinical trial information: NCT02601937