Patient description: The patient is a 37-year-old woman who came to the clinic for a routine Pap smear. The test revealed stage 1B of cervical cancer. The patient tested positive for HPV 16. She has three sexual partners and four children. She underwent tubal ligation. She smokes 30 packs/year and drinks alcohol frequently. She is otherwise healthy. She was offered a radical hysterectomy.

Clinical trial eligibility criteria: eligible ages (years): 0.5-17.0,  Rhabdoid Tumors INI1-negative Tumors Synovial Sarcoma Malignant Rhabdoid Tumor of Ovary Age (at the time of consent/assent): ≥6 months to <18 years Cohort 4 only: ≥10 years to <18 years 2. Performance Status If <12 years of age: Lanksy Performance Status >50% If ≥12 years of age: Karnofsky Performance Status >50% NOTE: If subject is unable to walk due to paralysis, but is mobile in a wheelchair, subject is considered to be ambulatory for the purpose of assessing their performance status. 3. Has provided signed written informed consent/assent 4. Has a life expectancy of >3 months 5. Has relapsed or refractory disease and no standard treatment options as determined by locally or regionally available standards of care and treating physician's discretion 6. Is ineligible or inappropriate for other treatment regimens known to have effective potential 7. Has a documented local diagnostic pathology of original biopsy confirmed by a Clinical Laboratory Improvement Amendments (CLIA)/College of American Pathologists (CAP) or equivalent laboratory certification 8. Has all prior treatment (i.e., chemotherapy, immunotherapy, radiotherapy) related clinically significant toxicities resolve to ≤ Grade 1 per CTCAE, version 4.03 or are clinically stable and not clinically significant, at time of enrollment 9. Has completed a prior therapy (ies) according to the below Other investigational study agent (any medicinal product that is not approved in the country of treatment for any indication, adult or pediatric) (At least 30 days or five half-lives, whichever is longer, since last dose prior to the first dose of tazemetostat) Chemotherapy: cytotoxic (At least 14 days since last dose of chemotherapy prior to first dose of tazemetostat) Chemotherapy: nitrosoureas (At least 6 weeks since last dose of nitrosoureas prior to first dose of tazemetostat) Chemotherapy: non-cytotoxic (e.g., small molecule inhibitor) (At least 14 days since last dose of non-cytotoxic chemotherapy prior to first dose of tazemetostat) Monoclonal antibody (ies) (At least 28 days since the last dose of any monoclonal antibody prior to first dose of tazemetostat) Immunotherapy (e.g., tumor vaccine) At least 6 weeks since last dose of immunotherapy agent(s) prior to first dose of tazemetostat)  Has had prior exposure to tazemetostat or other inhibitor(s) of EZH2 2. Is being actively treated for another concurrent malignancy or is less than five years from completion of treatment for another malignancy 3. Has participated in another interventional clinical study and received investigational drug within 30 days or 5 half-lives, whichever is longer, prior to the planned first dose of tazemetostat 4. Has had major surgery within 2 weeks prior to enrollment NOTE: Minor surgery (e.g., minor biopsy of extracranial site, central venous catheter placement, shunt revision) is permitted within 2 weeks prior to enrollment. 5. Has thrombocytopenia, neutropenia, or anemia of Grade ≥3 (per CTCAE 4.03 criteria) or any prior history of myeloid malignancies, including myelodysplastic syndrome (MDS). Has abnormalities known to be associated with MDS (e.g. del 5q, chr 7 abn) and MPN (e.g. JAK2 V617F) observed in cytogenetic testing and DNA sequencing. Note: Bone marrow aspirate/biopsy will be conducted following abnormal peripheral blood smear morphology assessment conducted by central lab at screening. Cytogenetic testing and DNA sequencing will be conducted following an abnormal result of bone marrow aspirate/biopsy. 6. Has a prior history of T-LBL/T-ALL. 7. Has clinically active heart disease including prolonged corrected QTcF (>450 msec) 8. Is currently taking any prohibited medication(s) as described in Section 7.3. 9. Is unwilling to grapefruit juice, Seville oranges and grapefruit from the diet and all foods that contain those fruits from time of enrollment to while on study 10. Has an active infection requiring systemic treatment 11. Is immunocompromised (i.e. congenital immunodeficiencies), including subjects known history of infection with human immunodeficiency virus (HIV) 12. Has known history of chronic infection with hepatitis B virus (hepatitis B surface antigen positive) or hepatitis C virus (detectable HCV RNA) 13. Has had a symptomatic venous thrombosis within the 14 days prior to study enrollment NOTE: Subjects with a history of a deep vein thrombosis 14 days prior to study enrollment who are on anticoagulation therapy with low molecular weight heparin are eligible for this study 14. For subjects with CNS involvement (primary tumor or metastatic disease): Have any active bleeding, or new intratumoral hemorrhage of more than punctate size on Screening MRI obtained within 14 days of starting study drug,or known bleeding diathesis or treatment with anti-platelet or anti-thrombotic agents 15.15. Has known hypersensitivity to any of the components of tazemetostat or other inhibitor(s) of EZH2, or hypersensitivity to Ora-sweet or methylparaben 16. Has an uncontrolled intercurrent illness including, but not limited to, uncontrolled infection, or psychiatric illness/social situations that would limit compliance with study requirements 17. For female subjects of childbearing potential: Is pregnant or nursing For male subjects: Is unwilling to adhere to contraception from time of enrollment in study to at least 30 days after last dose of tazemetostat