Inclusion Criteria:

          1. Subject must be ≥ 18 or ≥ 16 years of age with a minimum body weight of 50 kg.

          2. Subject must have a diagnosis of SS, defined by the presence of the SS18-SSX
             rearrangement, as confirmed by the Investigator (evidence from the diagnostic
             pathology of prior biopsy must be available). Subject must have advanced SS, which for
             the purposes of this study, is defined as any of the following:

               -  Metastatic

               -  Local (primary or recurrent), unresectable (with Investigator and Medical Monitor
                  approval)

             Subject must have been treated with ≤ 4 regimens of systemic chemotherapies. Subjects
             who have undergone > 4 regimens of systemic chemotherapy may be permitted with Medical
             Monitor approval. Subjects must have:

               -  Demonstrated progression of disease on their most recent therapy or

               -  Discontinued their most recent therapy due to the potential for cumulative
                  toxicity, intolerability or lack of continued clinical benefit, in the opinion of
                  the Investigator.

             Eligible subjects with progression of disease on their most recent therapy may enroll
             in the Dose Escalation Phase and in Arm 1 of the Dose Expansion Phase. Eligible
             subjects with responsive and/or stable disease on their most recent therapy may enroll
             in the Dose Escalation Phase and in Arm 2 of the Dose Expansion Phase.

             Note: Inclusion criterion 15 provides timing requirements for prior therapy.

          3. Subject must have measurable disease by RECIST v1.1, defined as at least one lesion
             that can be accurately measured in at least one dimension (longest diameter to be
             recorded) as ≥ 10 mm with calipers and/or CT scan. Measurable lesions cannot have
             undergone any local treatment or radiation nor can any local treatment or radiation
             involving measurable lesions be anticipated.

          4. Subject or his/her parent or legal guardian (when applicable) must be able to
             understand and be willing to sign an informed consent and, when applicable, subject
             must sign assent form.

          5. Subject must be willing and able to comply with scheduled study visits and treatment
             plans.

          6. Subject must be willing to undergo all study procedures (biopsies at baseline, at
             least 1 on-treatment and at EOT [unless contraindicated due to medical risk; other
             exceptions to this are at the discretion of the Sponsor's Medical Monitor]),
             laboratory testing, and imaging approximately every 8 (or 12) weeks independent of
             dose delays, interruptions, and/or reductions.

          7. Subject must have an ECOG PS of ≤ 2.

             • Arm 2 (Dose Expansion Phase): Subject must have an ECOG PS of ≤ 3

          8. Subject must have a life expectancy of ≥ 3 months.

             • Arm 2 (Dose Expansion Phase): Subject must have a life expectancy of ≥ 2 months

          9. Subject must have adequate venous access for IV drug administration and blood
             collection.

         10. Subject must have adequate cardiac function as evidenced by:

               -  LVEF of ≥ 40% by ECHO. Other methods of evaluating LVEF may be performed
                  according to institutional practice.

               -  Corrected QT interval (QTc) using Fridericia's formula (QTcF) < 470 msec

         11. Subject must have adequate hepatic function as evidenced by:

               -  Serum total bilirubin ≤ 1.5 × upper limit of normal (ULN) (≤ 3.0 × ULN for
                  subjects with Gilbert's syndrome)

               -  Aspartate aminotransferase (AST), alanine aminotransferase (ALT) ≤ 3.0 × ULN (≤
                  5.0 × ULN if liver metastases are present)

               -  Alkaline phosphatase (ALP) ≤ 3.0 × ULN (≤ 5.0 × ULN if liver metastases are
                  present and/or known bone disease is present)

               -  No known portal vein thrombosis

         12. Subject must have adequate renal function as evidenced by:

             • Glomerular filtration rate (GFR) ≥ 50 mL/min (based on a contemporary, widely
             accepted, and clinically applicable equation that estimates glomerular filtration rate
             or a measure of glomerular filtration rate (e.g. Chronic Kidney Disease Epidemiology
             Collaboration CKD-EPI)

         13. Subject must have adequate hematologic function as evidenced by:

               -  Hemoglobin ≥ 8 g/dL (Red blood cell [RBC] transfusions to achieve this level will
                  be permitted up to 7 days prior to start of study drug and complete blood count
                  [CBC] criteria for eligibility are confirmed within 24 hours of first study
                  dose.)

               -  White blood cells (WBCs) ≥ 2.0 × 109/L

               -  Absolute neutrophil count (ANC) ≥ 1.0 × 109/L

               -  Platelets > 50 × 109/L (Transfusions to achieve this level will be allowed up to
                  72 hours prior to start of study drug.)

         14. Subject must have adequate coagulation function as evidenced by:

             • International normalized ratio (INR) ≤ 1.5 or prothrombin time (PT) ≤ 1.5 × ULN and
             partial thromboplastin time (PTT) ≤ 1.5 × ULN if not receiving anticoagulation
             therapy.

             Note: For subjects on anticoagulants, exceptions to these parameters are allowed if
             they are within the intended or expected range for their therapeutic use. Participants
             must have no history of clinically significant active bleeding (within 14 days of
             first dose of study drug) or pathological condition that carries high risk of bleeding
             (for example, tumor involving major vessels or known esophageal varices).

         15. Timing requirements with respect to prior therapy and surgery are as follows:

               -  At least 2 weeks or at least 5 half-lives, whichever is shorter, must have
                  elapsed since administration of the last dose of any prior anticancer therapy
                  (including investigational agents).

               -  4 weeks must have elapsed since the last major surgery, laparoscopic procedure,
                  or significant traumatic injury. Note: Central line placement, subcutaneous port
                  placement, core biopsy, fine needle aspiration, and bone marrow biopsy/aspiration
                  are not considered major surgeries.

               -  2 weeks must have elapsed since the last radiotherapy. Palliative radiation
                  therapy is allowed so long as it does not involve the target lesion(s).

         16. Toxicity related to prior therapy must have returned to ≤ Grade 1 by CTCAE at least 14
             days prior to study start. Exceptions include Grade 2 alopecia and other Grade 2
             toxicities determined to be stable and irreversible by the Investigator with approval
             of the Medical Monitor.

         17. Female subjects must be:

               -  Postmenopausal, defined as at least 12 months post-cessation of menses (without
                  an alternative medical cause); or

               -  Permanently sterile following documented hysterectomy, bilateral salpingectomy,
                  bilateral oophorectomy, or tubal ligation or having a male partner with vasectomy
                  or having a female partner as affirmed by the subject; or

               -  Nonpregnant, nonlactating, and if sexually active having agreed to use a highly
                  effective method of contraception (ie, hormonal contraceptives associated with
                  inhibition of ovulation or intrauterine device [IUD], or intrauterine
                  hormone-releasing system [IUS], or sexual abstinence) from Screening Visit until
                  90 days after final dose of study drug.

         18. Male subjects must have documented vasectomy or if sexually active must agree to use a
             highly effective method of contraception with their partners of childbearing potential
             (ie, hormonal contraceptives associated with the inhibition of ovulation or IUD, or
             IUS, or sexual abstinence) from Screening until 90 days after final dose of study
             drug. Male subjects must agree to refrain from donating sperm during this time period.

        Exclusion Criteria:

          1. Subject (or his/her parent or legal guardian, when applicable) is unable to provide
             informed consent (or assent, when applicable) and/or to follow protocol requirements.

          2. Subject has other malignancy which may interfere with the diagnosis and/or treatment
             of SS and/or interpretation of outcome results.

          3. Subject has an active severe infection requiring systemic therapy. Subject is
             permitted to enroll once any required antibiotic and/or antifungal therapy has been
             completed and/or infection is determined to be controlled.

          4. Subject has active hepatitis B virus (HBV) or hepatitis C virus (HCV) infections;
             subjects with a sustained viral response to HCV treatment or immunity to prior HBV
             infection will be permitted. Subject has known positive human immunodeficiency virus
             (HIV) antibody results or acquired immunodeficiency syndrome (AIDS)-related illness;
             subjects with CD4+ T-cell counts ≥ 350 cells/µL will be permitted, as will subjects
             who have not had an AIDS-related illness within the past 12 months.

          5. Subject has an uncontrolled concurrent medical disease and/or psychiatric
             illness/social situation that in the opinion of the Investigator could cause
             unacceptable safety risks or compromise compliance with the protocol.

          6. Subject is receiving systemic steroid therapy for acute illness (stable doses for
             controlled chronic disease are permitted) or any other systemic immunosuppressive
             medication. Local steroid therapies (inhaled or topical steroids) are acceptable. See
             Exclusion criterion 7 for details on steroids in the setting of central nervous system
             (CNS) disease.

          7. Subjects with known CNS metastases are only permitted under the following conditions:
             Brain metastases must have been stable for the at least 2 months since completion of
             most recent CNS-directed intervention. Subject may be on corticosteroids so long as
             the dose is stable or decreasing at the time of study entry. Anti-epileptic therapy is
             allowed so long as medications are not otherwise excluded and seizures have been
             controlled for at least 4 weeks since the last anti-epileptic medication adjustment.
             Subjects with active brain metastases and/or leptomeningeal disease are excluded.

               -  Dose Escalation Phase: Subjects with known CNS metastases that meet the above
                  conditions are permitted to enroll in dose escalation.

               -  Arm 1 (Dose Expansion Phase): Subjects with known or suspected CNS metastases are
                  excluded from Arm 1.

               -  Arm 2 (Dose Expansion Phase): Subjects with CNS metastases that meet the above
                  conditions are permitted to enroll in Arm 2.

          8. Subject has known hypersensitivities to components of FHD-609.

          9. Subject has prior exposure to a BRD9 degrader.

         10. Subject is participating in any other clinical trials. Exceptions include
             participation in any observational or nontherapeutic clinical trials.