Brief Summary: This phase Ib/II trial is aimed at studying the combination of a drug named Selinexor (selective inhibitor of nuclear export) in combination with standard therapy for B cell Non-Hodgkin's lymphoma called R-CHOP. The investigators will establish maximum tolerated dose of Selinexor in combination with RCHOP and also study the efficacy of this combination for therapy of B cell Non-Hodgkin's lymphoma. Giving Selinexor plus chemotherapy may work better in treating patients with B cell non-Hodgkin lymphoma.

Detailed Description: The study will be done in two phases namely phase 1B and phase 2. In the phase 1B component the investigators intend to enroll patients in a 3+3 dose escalation design. Newly diagnosed indolent and diffuse large cell lymphomas as well as relapsed/refractory indolent B cell lymphomas are eligible for enrollment in the phase 1 component. The primary end-point for this component would be to establish the recommended phase 2 dose (RP2D) for Selinexor in combination with standard dose RCHOP chemotherapy. In the phase 2 part of the study the investigators will use recommended phase 2 dose of Selinexor plus standard dose RCHOP combination to treat newly diagnosed DLBCL patients with the primary end-point being 2 year Progression free survival. Maintenance Phase: Patients with Follicular Lymphoma and Diffuse Large B cell lymphoma able to achieve PR or better at the end of therapy scan will be put on maintenance Selinexor for a total of one year. The dose of Selinexor in the maintenance phase would be similar to the last dose used for that particular patient in the treatment phase 1 or 2.