 This is a Phase 2b, single-arm, open-label, multicenter study of selinexor 80 mg plus dexamethasone 20 mg Sd, both dosed twice weekly in each four-week cycle, in patients with MM previously treated with lenalidomide, pomalidomide, bortezomib, carfilzomib, and daratumumab and refractory to prior treatment with glucocorticoids, an immunomodulatory agent IMiD, a proteasome inhibitor PI, and daratumumab. 