Inclusion Criteria

        Subjects must meet ALL the following inclusion criteria to be eligible to enroll in this
        study:

          1. Have voluntarily agreed to provide written informed consent and demonstrated
             willingness and ability to comply with all aspects of the protocol. Study related
             activities will not start until written consent is obtained.

          2. Phase 1b: 18-65 years old histologically confirmed STS

          3. Phase 3: ≥18 years old with Epithelioid Sarcoma

          4. Have measurable disease

          5. Females must not be lactating or pregnant at Screening or Baseline (as documented by a
             negative pregnancy test)

          6. Because of unknown impact on a developing fetus, females must not be pregnant or
             breast feeding and agree to use highly effective contraception during the clinical
             trial and for 6 months following the final dose of study

          7. Male subjects of child-bearing potential must have had either a successful vasectomy
             or practice highly effective contraception and use a condom throughout the study
             period and for 3 months after doxorubicin or tazmetostat discontinuation.

          8. Subjects diagnosed with human immunodeficiency virus (HIV) are eligible to participate
             in the study if their infection is well controlled on anti-retroviral therapy.

        Exclusion Criteria

        Subjects meeting ANY of the following exclusion criteria are NOT eligible to enroll in this
        study:

          1. Prior exposure to tazemetostat or other inhibitor(s) of enhancer of zeste homologue-2
             (EZH2).

          2. Prior systemic anticancer therapy.

          3. Have any prior history of myeloid malignancies, including myelodysplastic syndrome
             (MDS) or acute myeloid leukemia (AML).

          4. Have prior history of T-cell lymphoblastic lymphoma/T-cell acute lymphoblastic
             leukemia (T- LBL/T-ALL).

          5. Have 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 study treatment.

          6. Have known active central nervous system (CNS) or any leptomeningeal metastasis of
             primary extracranial tumor.

          7. Subjects taking medications that are known potent cytochrome P450 (CYP)3A4
             inducers/inhibitors (including St. John's Wort)

          8. Are unwilling to exclude Seville oranges, grapefruit juice, AND grapefruit from the
             diet and all foods that contain those fruits from time of enrollment to through the
             duration of study participation.

          9. Major surgery within 4 weeks before the first dose of study treatment. Subjects must
             have recovered from surgery prior to enrollment to this study.

         10. Are unable to take oral medication OR have malabsorption syndrome or any other
             uncontrolled gastrointestinal condition (eg, nausea, diarrhea, vomiting) that might
             impair the bioavailability of study treatment.

         11. Have an active infection requiring systemic therapy.

         12. Are immunocompromised (ie, has a congenital immunodeficiency).

         13. Have known hypersensitivity to any component of tazemetostat or doxorubicin.

         14. Have a known active infection with hepatitis B virus (HBV, as measured by positive
             hepatitis B surface antigen), hepatitis C virus (HCV, as measured by positive
             hepatitis C antibody).

         15. Any other major illness that, in the Investigator's judgment, will substantially
             increase the risk associated with the subject's participation in this study OR
             interfere with their ability to receive study treatment or complete the study.

         16. Female subjects who are pregnant or breastfeeding.

         17. Subjects who have undergone a solid organ transplant.

         18. Subjects housed in an institution by order of the authorities or courts.