Inclusion Criteria:

          -  Patients must meet all of the following inclusion criteria to be eligible to enroll in
             this study:

               1. Age ≥ 18 year of age.

               2. Willing and able to provide written informed consent in accordance with federal,
                  local, and institutional guidelines. The patient must provide informed consent
                  prior to the first screening procedure.

               3. Able to adhere to the study visit schedule and other protocol requirements.

               4. ECOG Performance Status (PS) of ≤ 2.

               5. Has a diagnosis of MM based on standard criteria (9) as follows:

        Major criteria:

          1. Plasmacytomas on tissue biopsy.

          2. Bone marrow plasmacytosis (greater than 30% plasma cells).

          3. Monoclonal immunoglobulin spike on serum electrophoresis IgG greater than 3.5 g/dL or
             IgA greater than 2.0 g/dL or kappa or lambda light chain excretion greater than 1
             g/day on 24-hour urine protein electrophoresis.

        Minor criteria:

          1. Bone marrow plasmacytosis (10% to 30% plasma cells).

          2. Monoclonal immunoglobulin present but of lesser magnitude than given under major
             criteria.

          3. Lytic bone lesions.

          4. Normal IgM less than 50 mg/dL, IgA less than 100 mg/dL, or IgG less than 600 mg/dL.

             Any of the following sets of criteria will confirm the diagnosis of multiple myeloma:

             • any 2 of the major criteria

             • major criterion 1 plus minor criterion 2, 3, or 4

             • major criterion 3 plus minor criterion 1 or 3

             • minor criteria 1, 2, and 3, or 1, 2, and 4

        6. Currently has MM with measurable disease, defined as:

          -  a monoclonal immunoglobulin spike on serum electrophoresis of at least 0.5 g/dL and/or

          -  urine monoclonal protein levels of at least 200 mg/24 hours

          -  for patients without measurable serum and urine M-protein levels, an involved SFLC >
             100 mg/L or abnormal SFLC ratio 7. Currently has progressive MM: MM patients that are
             relapsed or have refractory disease from at least 3 regimens or lines of therapy are
             eligible for enrollment provided they fulfill the other eligibility criteria:

          -  patients are considered relapsed, when they progress greater than 8 weeks from their
             last dose of treatment

          -  patients are refractory when they progress while currently receiving the treatment or
             within 8 weeks of its last dose 8. Previous exposure to lenalidomide: failed
             lenalidomide (> 10 mg)-containing regimen (lenalidomide- containing regimen could be
             any prior regimen and is not required to be a part of their most recent treatment) and
             have received a proteasome inhibitor, an immunomodulatory agent and an anti-CD38
             antibody therapy.

             9. Adequate hepatic function within 28 days prior to C1D1: Total bilirubin < 1.5 ×
             upper limit of normal (ULN) (except patients with Gilbert's syndrome who must have a
             total bilirubin of < 3 × ULN), and Aspartate aminotransferase (AST) and alanine
             aminotransferase (ALT) normal to < 2 × ULN.

             10. Adequate renal function within 28 days prior to C1D1 as determined by serum
             creatinine of ≤1.5 mg/dL OR estimated CrCl of > 60 mL/min, calculated using the
             Cockcroft and Gault formula (140 - Age) • Mass (kg)/ (72 • creatinine mg/dL); multiply
             by 0.85 if female (10)(Appendix 5).

             11. Adequate hematopoietic function within 7 days prior to C1D1: total WBC count
             ≥1500/mm3, ANC ≥1000/mm3, hemoglobin ≥8.5 g/dL and platelet count ≥75,000/mm3
             (patients for whom <50% of BM nucleated cells are plasma cells) or ≥50,000/mm3
             (patients for whom ≥50% of BM nucleated cells are plasma cells).

             12. Patients receiving hematopoietic growth factor support, including erythropoietin,
             darbepoetin, G-CSF, GM-CSF, and platelet stimulators (e.g., eltrombopag, romiplostim,
             or interleukin-11) must have a 2-week interval between growth factor support and the
             Screening assessments, but they may receive growth factor support during the study.

             13. Patients must have:

          -  At least a 2-week interval from the last red blood cell (RBC) transfusion prior to the
             start of study treatment

          -  At least a 1-week interval from the last platelet transfusion prior to the start of
             study treatment

          -  However, patients may receive RBC and/or platelet transfusions as clinically indicated
             per institutional guidelines during the study 14. Patients must be registered into the
             mandatory REVLIMID REMS™ program and be willing and able to comply with the
             requirements of the REVLIMID REMS™ program.

             15. Female patients of childbearing potential (FCBP) must have a negative serum
             pregnancy test at Screening. Female patients of childbearing potential and fertile
             male patients who are sexually active with a female of childbearing potential must use
             highly effective methods of contraception throughout the study and for 3 months
             following the last dose of study treatment, specifically:

          -  FCBP† must have a negative serum or urine pregnancy test with a sensitivity of at
             least 25 mIU/mL within 10 - 14 days prior to and again within 24 hours of starting
             treatment and must either commit to continued abstinence from heterosexual intercourse
             or use acceptable methods of birth control, one highly effective method and one
             additional effective method AT THE SAME TIME, and at least 28 days before she starts
             therapy. FCBP must also agree to ongoing pregnancy testing. Men must agree to use a
             latex condom during sexual contact with a FCBP even if they have had a vasectomy. All
             subjects must be counseled at a minimum of every 28 days about pregnancy precautions
             and risks of fetal exposure.

               -  A FCBP (female of childbearing potential) is a sexually mature woman who: 1) has
                  not undergone a hysterectomy or bilateral oophorectomy; or 2) has not been
                  naturally postmenopausal for at least 24 consecutive months (i.e., has had menses
                  at any time in the preceding 24 consecutive months) Able to take aspirin
                  (acetylsalicylic acid, ASA) at 81 or 325 mg/daily as antiplatelet therapy if
                  platelet count is above 30 x 109/L (subjects intolerant to ASA may use warfarin
                  or low molecular weight heparin)

        Exclusion Criteria:

          -  Patients meeting any of the following exclusion criteria are not eligible to enroll in
             this study:

               1. Has received selinexor or another XPO1 inhibitor previously.

               2. Prior malignancy that required treatment or has shown evidence of recurrence
                  (except for non-melanoma skin cancer or adequately treated cervical carcinoma in
                  situ) during the 5 years prior to randomization. Cancer treated with curative
                  intent for >5 years previously and without evidence of recurrence will be
                  allowed.

               3. Has any concurrent medical condition or disease (e.g., uncontrolled active
                  hypertension, uncontrolled active diabetes, active systemic infection, POEMS
                  syndrome [polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and
                  skin changes], primary amyloidosis, etc.) that is likely to interfere with study
                  procedures.

               4. Uncontrolled active infection requiring parenteral antibiotics, antivirals, or
                  antifungals within 1 week prior to Cycle 1 Day 1 (C1D1). Patients on prophylactic
                  antibiotics or with a controlled infection within 1 week prior to C1D1 are
                  acceptable.

               5. Known intolerance, hypersensitivity, or contraindication to glucocorticoids.

               6. Known hypersensitivity to compounds of similar chemical or biological composition
                  to thalidomide and lenalidomide or steroids.

               7. Concurrent use of other anti-cancer agents or treatments.

               8. The development of erythema nodosum if characterized by a desquamating rash while
                  taking thalidomide or similar drugs.

               9. Severe hypercalcemia, i.e., serum calcium ≥ 12 mg/dL (3.0 mmol/L) corrected for
                  albumin

              10. Any condition, including the presence of laboratory abnormalities, which places
                  the subject at unacceptable risk if he/she were to participate in the study or
                  confounds the ability to interpret data from the study.

              11. Pregnant or breastfeeding females.

              12. BSA <1.4 m2 at baseline, calculated by the Dubois (58) or Mosteller (59) method.

              13. Life expectancy of less than 3 months.

              14. Major surgery within 4 weeks prior to C1D1.

              15. Active, unstable cardiovascular function, as indicated by the presence of:

             1. Symptomatic ischemia, or 2. Uncontrolled clinically significant conduction
             abnormalities (e.g., patients with ventricular tachycardia on anti-arrhythmic are
             excluded; patients with first degree atrioventricular block or asymptomatic left
             anterior fascicular block/right bundle branch block will not be excluded), or 3. CHF
             of New York Heart Association Class ≥3 or known left ventricular ejection fraction <
             40%, or 4. MI within 3 months prior to C1D1. 16. Known active HIV infection or HIV
             seropositivity. 17. Known active hepatitis A, B, or C infection; or known to be
             positive for hepatitis C virus ribonucleic acid (RNA) or hepatitis B virus surface
             antigen.

             18. Any active GI dysfunction interfering with the patient's ability to swallow
             tablets, or any active GI dysfunction that could interfere with absorption of study
             treatment.

             19. Inability or unwillingness to take supportive medications such as anti-nausea and
             anti-anorexia agents as recommended by the National Comprehensive Cancer Network®
             (NCCN) Clinical Practice Guidelines in Oncology (CPGO) (NCCN CPGO) for antiemesis and
             anorexia/cachexia (palliative care).

             20. Any active, serious psychiatric, medical, or other conditions/situations that, in
             the opinion of the Investigator, could interfere with treatment, compliance, or the
             ability to give informed consent.

             21. Contraindication to any of the required concomitant drugs or supportive
             treatments.

             22. Patients unwilling or unable to comply with the protocol.