Inclusion Criteria:

          -  Willing and able to provide written Institutional Review Board (IRB)/Independent
             Ethics Committee (IEC)-approved informed consent. For subjects < 18 years of age,
             their parents or legal guardians must sign a written informed consent. Assent, when
             appropriate, will be obtained according to institutional guidelines

          -  Have histologically or cytological confirmed diagnosis of rhabdomyosarcoma with
             locally advanced/unresectable, metastatic, refractory or relapsed disease who have
             failed standard therapy and for whom no known curative therapy exists

          -  Measurable disease according to RECIST version 1.1

          -  Prior cancer therapy: Subjects may have received any number of prior therapy regimens.
             In the investigator's opinion, subjects must have tolerated prior cytotoxic therapies
             well and have adequate bone marrow reserve. At the time of treatment initiation, at
             least 3 weeks must have elapsed after prior cytotoxic chemotherapy. At least 7 days
             must have elapsed since completion of any prior non-cytotoxic cancer therapy and any
             associated AEs must have resolved

          -  Prior radiotherapy is allowed if >= 2 weeks have elapsed for local palliative
             radiation therapy (XRT) (small port); >= 6 months must have elapsed if prior total
             body irradiation, craniospinal XRT or if > 50% radiation of the pelvis; > 6 weeks must
             have elapsed if other substantial bone marrow radiation (defined per principal
             investigator's [PI's] discretion). Subjects who have received brain irradiation must
             have completed whole brain radiotherapy and/or gamma knife at least 4 weeks prior to
             enrollment

          -  Subjects with controlled asymptomatic central nervous system (CNS) involvement are
             allowed in absence of therapy with anticonvulsants. Subjects not requiring steroids or
             requiring steroids at a stable dose (=< 4 mg/day dexamethasone or equivalent) for at
             least 2 weeks are eligible

          -  Resolution of all acute toxic effects (excluding alopecia) of any prior anti-cancer
             therapy to National Cancer Institute (NCI) Common Terminology Criteria for Adverse
             Events (CTCAE) (version 4.03) grade < 1 or to the baseline laboratory values as
             defined below

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS) =< 2 in subjects >=
             17 years old; or Karnofsky/Lansky > 50 in subjects < 16 years old

          -  Subjects age > 18 years for first cohort. Subjects must be > 12 years old for the
             second and subsequent cohorts

          -  Life expectancy of at least 3 months

          -  Absolute neutrophil count (ANC) >= 1000/mm^3 (>= 1.0 x 10^9/L)

          -  Platelets (PLT) >= 100,000/mm^3 (>= 100 x 10^9/L) (transfusion independent, defined as
             not receiving platelet transfusions within a 7 day period prior to screening)

          -  Hemoglobin > 9.0 g/dL (transfusions are allowed)

          -  Serum creatinine =< 1.5 x upper limit of normal (ULN) or creatinine clearance > 60
             mL/min

          -  Total serum bilirubin =< 1.5 x ULN; =< 5 x ULN if Gilbert's syndrome

          -  Liver transaminases (aspartate aminotransferase [AST]/alanine aminotransferase [ALT])
             =< 2.5 x ULN; =< 5 x ULN if liver metastases are present

          -  Pregnancy test if female of child-bearing potential negative within 7 days of starting
             treatment

          -  Cardiac ejection fraction > 50% or shortening fraction > 28% by echocardiography
             (ECHO) or multigated acquisition scan (MUGA)

          -  Females of child-bearing potential must have a negative pregnancy test during
             screening and be neither breastfeeding nor intending to become pregnant during study
             participation. Females of childbearing potential must agree to avoid pregnancy during
             the study and commit to abstinence from heterosexual intercourse or agree to use two
             methods of birth control (one highly effective method and one additional effective
             method) at least 4 weeks before the start of protocol therapy, for the duration of
             study participation, and for 6 months after the last dose of mocetinostat

          -  Males with partner(s) of childbearing potential must take appropriate precautions to
             avoid fathering a child from the screening period until 90 days after receiving the
             last dose of mocetinostat. They must commit to abstinence from heterosexual
             intercourse or agree to use appropriate barrier contraception

          -  Prior to enrollment of females or males of reproductive potential, the investigator
             must document confirmation of the subject's understanding of the possible teratogenic
             effects of mocetinostat

          -  Willingness and ability to comply with scheduled visits, treatment plan, laboratory
             tests and other study procedures

        Exclusion Criteria:

          -  Current participation in another therapeutic clinical trial

          -  Symptomatic brain metastases

          -  History of previous cancer (non RMS), except squamous cell or basal-cell carcinoma of
             the skin or any in situ carcinoma that has been completely resected, which required
             therapy within the previous 3 years. Other low grade cancers can be reviewed and
             allowed at the discretion of the PI

          -  Incomplete recovery from any surgery (other than central venous catheter or port
             placement) prior to treatment

          -  Any of the following in the past 6 months: pericarditis, pericardial effusion,
             symptomatic congestive heart failure, cerebrovascular accident or transient ischemic
             attack, pulmonary embolism, deep vein thrombosis, symptomatic bradycardia, requirement
             for anti-arrhythmic medication

          -  History of prolonged corrected QT (QTc) interval (e.g., repeated demonstration of a
             QTc interval > 450 milliseconds, unless associated with the use of medications known
             to prolong the QTc interval). QTc will be calculated using the Bazett formula (RR
             interval = 60/heart rate; QTI corrected = QT interval/sqr[RRinterval])

          -  History of additional risk factors for torsade de pointes (e.g., heart failure, family
             history of long QT syndrome)

          -  Use of concomitant medications that increase or possibly increase the risk to prolong
             the QTc interval and/or induce torsades de pointes ventricular arrhythmia

          -  Females who are breastfeeding/lactating

          -  Known active infections (e.g., bacterial, fungal, viral including hepatitis and human
             immunodeficiency virus [HIV] positivity)

          -  Other severe acute or chronic medical or psychiatric condition or laboratory
             abnormality that may increase the risk associated with study participation or study
             drug administration or may interfere with the interpretation of study results and, in
             the judgment of the investigator, would make the subject inappropriate for entry into
             this study or compromise protocol objectives in the opinion of the investigator and/or
             the sponsor