Patients must have a body surface area (BSA) >= 0.84 m^2 Body surface area (BSA) must be > 0.67 and < 2.5 m^2 Body surface area (BSA)\r\n* Patients enrolled on dose level -1 must have BSA >= 0.55m^2\r\n* Patients enrolled on dose level -0.5 must have BSA >= 0.75m^2\r\n* Patients enrolled on dose level 0 must have BSA >= 0.55m^2\r\n* Patients enrolled on dose level 1 must have BSA >= 0.75m^2\r\n* Patients enrolled on dose level 2 and 3 must have BSA >= 0.45m^2 BSA\r\n* Patients enrolled on dose level -1 must have BSA >= 0.55m2\r\n* Patients enrolled on dose level -0.5 must have BSA >= 0.75m2\r\n* Patients enrolled on dose level 0 must have BSA >= 0.55m2\r\n* Patients enrolled on dose level 1 must have BSA >= 0.75m2\r\n* Patients enrolled on dose level 2 and 3 must have BSA >= 0.45m2 Body surface area (BSA) >= 0.5 m^2 Body surface area (BSA) ? 1.2 m2 Patients who will be receiving the tablet formulation must have a body surface area (BSA) >= 0.84 m^2 (square meter) at baseline. Specific body surface area (BSA) criteria depending on enrolling dose level: no restrictions for dose level 0, BSA >= 0.75 required for dose levels -1 and -2, BSA > 0.90 required for dose levels -3 and -4 BSA (m2) of <0.5 Body surface area (BSA):\r\n* Patients enrolled on dose level 1 (50 mg/m^2) must have BSA >= 1.20 m^2\r\n* Patients enrolled on dose level 2 (75 mg/m^2) must have BSA >= 0.93 m^2\r\n* Patients enrolled on dose level 3 (95 mg/m^2) must have BSA >= 0.70 m^2 Patients with a BSA ? 1.17 m^2 at time of study enrollment are not eligible Patients must have a body surface area (BSA) >= 0.55 m^2 Body surface area (BSA) greater than 0.3 m^2 Body surface area (BSA) >= 0.55 m^2 All races and ethnic groups will be included; for subjects between the ages of 12-18 years only, body surface area (BSA) must be >= 1.28 m^2 Body surface area requirements varied by dose level: \r\n* Dose level: -1; body surface area (BSA): >= 0.82 m^2\r\n* Dose level: 1; BSA: >= 0.66 m^2\r\n* Dose level: 2; BSA: >= 0.52 m^2\r\n* Dose level: 3; BSA: >= 0.45 m^2 BSA (m2) of <0.25 For subjects between the ages of 12-18 years only, body surface area (BSA) must be >= 1.5 m^2 Patients with a body surface area (BSA) =< 0.4 m^2 are excluded Rash must cover less than 10% of body surface area (BSA) Body surface area: the minimum body surface area (BSA) allowed for enrollment at dose level 1 to 0.85 m^2; the minimum for dose level 2 is BSA = 0.6 m^2 and the minimum for dose level 3 is BSA = 0.42 m^2 To ensure that no patient will receive a dose of selinexor > 70 mg/m^2, body surface area (BSA) calculated by Dubois method must be > 1.43 m^2 Patients must have body surface area (BSA) > 0.55 m^2 at the time of enrollment\r\n* In the event of de-escalation from dose level 1 to dose level 0, patients with BSAs < 0.67 m^2 are not eligible Serum creatinine =< 1.5 x the institutional upper limits of normal or corrected creatinine clearance of >= 50 mg/ml/hr/1.73 m^2 body surface area (BSA) Body surface area (BSA) greater than 0.3 m^2 Body surface area (BSA) >= 0.5 m^2 Participants must have a minimum body surface area (BSA) of 0.6 m^2 at study entry, with the exception of one study arm which requires a minimum BSA of 1.33 m^2. Obtained within 14 days prior to registration: Serum creatinine =< 1.5 x the institutional upper limits of normal or corrected creatinine clearance of >= 50 mg/ml/hr/1.73 m^2 body surface area (BSA) Body surface area (BSA) >= 1.04 m^2 Rash must cover less than 10% of body surface area (BSA) RECURRENT/ PROGRESSIVE DIPG (STRATUM 1): Body surface area (BSA) \r\n* Patients must have a BSA >= 0.80 m^2 for dose 5 mg/m^2\r\n* Patients must have a BSA >= 0.65 m^2 for doses of 10 mg/m^2 - 22 mg/m^2 \r\n* Patients must have a BSA >= 0.50 m^2 for doses of 28 mg/m^2 - 36 mg/m^2 NON-PROGRESSED DIPG (STRATUM 2): BSA\r\n* Patients must have a BSA >= 0.80 m^2 for dose 5 mg/m^2\r\n* Patients must have a BSA >= 0.65 m^2 for doses of 10mg/m^2 - 22 mg/m^2\r\n* Patients must have a BSA >= 0.50 m^2 for doses of 28 mg/m^2 - 36 mg/m^2 Body surface area (BSA) ? 2.4 m2. Patients age 16-17 years are eligible only if they have a body surface area (BSA) >= 1.7 m^2 or weigh >= 60 kg PHASE II: >= 2 and =< 18 years; body surface area (BSA) >= 0.55 m^2, and able to swallow whole capsules All patients must have an accurate pre-op height and weight and BSA (body surface area) calculation for CO (cardiac output) monitoring on FloTrac BSA (Body Surface Area) of <0.25 m2. BSA (m2) of <0.25 Body Surface Area (BSA) (m2) of <0.25 Subjects whose body surface area (BSA) would expose them to < 75% or > 125% of the target dose