A Study of Selinexor Monotherapy in Subjects With JAK Inhibitor-naïve Myelofibrosis and Moderate Thrombocytopenia

Key Inclusion Criteria:

* A diagnosis of MF or post-ET or post-PV MF according to the 2016 World Health Organization (WHO) classification of MPN, confirmed by the most recent local pathology report
* Measurable splenomegaly during the screening period as demonstrated by spleen volume of greater than or equal to (\>=) 450 cubic square centimeter (cm\^3) by MRI or CT scan (results from MRI or CT imaging performed within 28 days prior to C1D1 are acceptable)
* DIPSS risk category of intermediate-1 with symptoms, or intermediate-2, or high-risk
* ECOG Performance Status less than or equal to (\<=) 2
* Platelet count of greater than or equal to (\>=) 50 x 10\^9/L without platelet transfusion within 7 days prior to the first dose of selinexor
* Absolute neutrophil count (ANC) \>=1.0 × 10\^9/L without need for growth factors within 7 days prior to the first dose of selinexor
* Adequate liver function as defined by the following: aspartate transaminase (AST) and alanine transaminase (ALT) \<= 2.5 × upper limit normal (ULN) and serum total bilirubin \<= 3×ULN
* Calculated creatinine clearance (CrCl) greater than (\>) 15 milliliter per minute (mL/min) based on the Cockcroft and Gault formula
* Active symptoms of MF as determined by presence of at least 2 symptoms with an average score \>= 5 or total score of \>= 12 at screening (at least 5 of 7 consecutive days immediately preceding C1D1) using the MFSAF V4.0
* Must provide bone marrow biopsy samples (samples obtained up to 3 months prior to C1D1 are permitted) at screening and during the study
* Currently not eligible for stem cell transplantation
* Must be willing to complete the MFSAF V4.0 daily during the study for evaluating the symptom response (i.e., TSS50)

Key Exclusion Criteria:

* More than 10% blasts in peripheral blood or bone marrow (accelerated or blast phase)
* Previous treatment with JAK inhibitors for MF
* Previous treatment with selinexor or other XPO1 inhibitors
* Females who are pregnant or lactating
* Prior splenectomy, splenic radiation, or a splenic embolization within 6 months prior to C1D1
* History of myocardial infarction, unstable angina, percutaneous transluminal coronary angioplasty (PTCA), coronary artery bypass graft (CABG), cerebrovascular accident (transient ischemic attack \[TIA\]), ventricular arrhythmias, congestive heart failure class \> 2 per New York Heart Association (NYHA) within 6 months of C1D1
* Unable to tolerate two forms of antiemetics prior to each dose for the first two cycles