Inclusion Criteria:

          -  Patients must have histologically confirmed aggressive B-cell non-Hodgkin lymphomas:

               -  DLBCL including ABC, GCB or PMBCL subtypes

               -  Double/triple hit lymphomas

               -  Indolent lymphomas transformed to aggressive lymphomas

               -  Follicular lymphomas grade 3B

          -  Patients must have received at least two cycles of anthracycline based chemotherapy
             administered with curative intent and one of the following:

               -  failed to have achieve at least a partial response after 2 or more cycles

               -  failed to achieve a complete response after 6 or more cycles

               -  progressed after an initial response

          -  Patients must be age ≥18 years.

          -  Patients must have at least one site of measurable disease, 1.5 cm in diameter or
             greater.

          -  Patients must have ECOG performance status of 0-2.

          -  Patients must have laboratory test results within these ranges:

               -  Absolute neutrophil count ≥ 1500/mm³

               -  Platelet count ≥ 100,000/mm³

               -  Serum creatinine clearance ≥40 mL/min

               -  Total bilirubin ≤ 1.5x ULN. Higher levels are acceptable if these can be
                  attributed to active hemolysis or ineffective erythropoiesis.

               -  AST (SGOT) and ALT (SGPT) ≤ 2x ULN

          -  Women of childbearing potential must agree to use dual methods of contraception and
             have a negative serum pregnancy test prior to selinexor treatment. Male patients must
             use an effective barrier method of contraception if sexually active with a female of
             child-bearing potential.

               -  Acceptable methods of contraception are condoms with contraceptive foam, oral,
                  implantable or injectable contraceptives, contraceptive patch, intrauterine
                  device, diaphragm with spermicidal gel, or a sexual partner who is surgically
                  sterilized or post-menopausal.

               -  For both male and female patients, effective methods of contraception must be
                  used throughout the study and for three months following the last dose.

          -  Patients must be able to understand and willing to sign a written informed consent
             document.

          -  Patients must be able to adhere to the study visit schedule and other protocol
             requirements.

          -  Patients must not have any serious medical condition, laboratory abnormality, or
             psychiatric illness that would prevent the subject from signing the informed consent
             form.

          -  Patients must not have 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.

        Exclusion Criteria:

          -  Patients with hyperuricemia or other potential signs of tumor lysis syndrome

          -  Patients with more than minimally symptomatic disease (i.e. > grade 1), high tumor
             burden, or other indication for urgent treatment.

          -  Patients who have had prior malignancies (other than B-cell lymphomas) for ≤5 years
             with exception of currently treated basal cell, squamous cell carcinoma of the skin,
             or carcinoma "in situ" of the cervix or breast.

          -  Patients who have had other anti-cancer therapy, including radiation or experimental
             drug or therapy, within 28 days of enrollment.

          -  Patients with known HIV, active hepatitis B, active hepatitis C.

          -  Patients with known central nervous system involvement by lymphoma.