Platelets >= 100,000/mm^3, or >= 50,000 in cases of ongoing bone marrow involvement (in either case, these must be independent of transfusion support), documented within 14 days of registration Absolute neutrophil count (ANC) <1,500/mm3, pre-transfusion platelets <100,000/mm3, or pre-transfusion hemoglobin <9.0 mg/dL (the patient is required to have at least 2 weeks free from blood transfusion, G-CSF and erythropoietin use prior to the hematology test) Platelets ?75,000/mm3 without platelet transfusion ? 3 days prior to C1D1 Platelets >= 100 x 10^9 (without platelet transfusion) Platelets >= 100,000/mm^3 transfusion independent (no platelet transfusion one week prior to enrollment) Platelets ? 50,000/mm^3 independent of transfusion support, with no active bleeding Platelets: >= 100,000/mm^3 or >= 50,000/mm^3 if known or suspected bone marrow involvement, independent of transfusion support in either situation Within two weeks prior to enrollment: Platelets >= 75,000/mcL and > 7 days since last platelet transfusion Platelets ? 100,000/?L without transfusion support within 28 days prior to study treatment Platelets > 100 x 10^9/L without transfusion Platelets >= 100,000/mcL (with or without transfusion) Patients who are receiving regularly scheduled blood (RBC) transfusion therapy (also termed chronic, prophylactic, or preventive transfusion) or have received a RBC transfusion for any reason within 60 days of signing the ICF Platelets >= 100,000 /mcL in the absence of transfusion support within 7 days of determining eligibility Platelets >= 50,000/mm^3 independent of transfusion support, with no active bleeding Platelets (no transfusion within prior 7 days) >= 100 K/cu mm Platelets >= 75,000/mcL (without transfusion support) Platelets >= 20,000/mcL or recovery to the baseline count (in the absence of transfusion), within 14 days prior to treatment Platelets >= 50,000/mcL in the absence of transfusion support within 7 days of determining eligibility within 16 days of treatment initiation Platelets: >= 100,000/mm^3 or >= 50,000/mm^3 if bone marrow involvement independent of transfusion support in either situation Platelets ? 100 K/mm^3 (transfusion to achieve this level is not permitted within 2 weeks of first study drug administration) Platelets >= 50 x 10^9/L (without transfusion support) Platelets >= 50,000/mcL (transfusion independent) Platelets >= 75,000/mm^3 without platelet transfusion =< 7 days prior to C1D1 Platelets >= 100,000/mcL (without transfusion within 2 weeks prior to cycle 1, day 1) Platelets >= 75,000/mcl (without transfusion for 7 days). Platelets >= 100,000/mcL (transfusion not permitted) Platelets >= 100,000/?L without transfusion support within the past 28 days Platelets greater than or equal to 75,000/mm^3 (transfusion independent; no transfusion for >= 7 days prior to study enrollment) Platelets > 50,000/cu mm (transfusion allowed - however patients must not require more than two platelet transfusions per week) Platelets > 100,000/ul (transfusion independent) Platelets >= 75,000/mcL (without transfusion support) Platelets >= 50,000 X 10^9 /L without transfusion if stem cells are not available Platelets > 100,000/ul (transfusion independent) Platelets >= 100,000/mm^3 (transfusion independent) Platelets > 100,000/ul (transfusion independent) Platelets > 100 x 10^9/L without transfusion and/or a bone marrow cellularity of >= 20% Platelets > 50 x 10^9/L without transfusion in the prior 7 days Platelets (no transfusion within prior 7 days) >= 100 K/cu mm Transfusion dependence, defined as having had transfusion (in the setting of active disease) of 2 or more units of RBC or platelets within 8 weeks prior to randomization. Within 28 days of study registration: Platelets >= 100,000 cells/mm^3 (independent of blood transfusion and/or growth factors within 3 months prior to registration) Platelets >= 100,000/ mm^3 (transfusion independent) Platelets >= 100,000/mcL (without transfusion) Within 14 days of subject registration: Platelets 100,000/uL (no transfusion allowed within 2 weeks) Platelets >= 70 x 10^9/L without platelet transfusion 7 days prior to study entry Platelets > 100 x 10^9/L without transfusion Patients must not have received a transfusion (platelets or red blood cells) within 4 weeks of the first dose of study treatment. Platelets >= 100,000/ul (no platelet transfusion allowed within 2 weeks) Performed within 14 days of patient registration: Platelets >= 100,000/uL (no transfusion allowed within 2 weeks) MF PATIENTS: Platelets >= 50,000/mm^3; Note: this must be achieved without transfusion Platelets ?100,000/mm3 (?100 × 109/L) without platelet transfusion for 7 days Platelets >100,000/mL (without platelet transfusion within the last 2 weeks) Platelets >= 75 x 10^9/L (no platelet transfusion within past 14 days) Need for transfusion within 14 days prior to the first dose of trial treatment Platelets > 100 k without transfusion support in the past 28 days Platelets: >= 100 x 10^9/L, or >= 50 x 10^9/L if bone marrow involvement and independent of transfusion support for 14 days in either situation Platelets >= 20,000 without platelet transfusion Platelets >= 100,000/mcL at all times during the screening period without platelet transfusion within 3 weeks Platelets >= 100 x 10^9/L and no platelet transfusion within the past 14 days prior to study registration Platelets >= 100,000 /mm^3; previous transfusion is allowed Platelets ? 80 × 10^9 cells/L (transfusion independent, defined as not receiving platelet transfusions within 7 days prior to laboratory sample). In the phase 2 portion, for patients with known bone marrow involvement, platelets ? 50 × 10^9 cells/L Platelets >= 100 x 10^9/L (tested within 14 days prior to registration)\r\n* Subjects may not have had a transfusion within 7 days of screening assessment Platelets >= 50,000/mcL in the absence of transfusion support within 7 days prior to determination of eligibility Platelets >= 100,000/ul (no transfusion allowed within 2 weeks) Platelets >= 100,000/mm^3 (transfusion independent) Platelets > 50,000 (platelet transfusion allowed) The patient has an absolute neutrophil count (ANC) ?1000/µL, platelets ?100,000/µL and is not transfusion dependent for platelets and/or red cells Platelets >= 100 x 10^9/L (without transfusion < 14 days before enrollment) Platelets >= 100 x 10^9/L (must be independent of platelet transfusion) Platelets >= 100,000/mcL (transfusion independent) within two weeks prior to enrollment Platelets ? 75,000/?L (without intervention, e.g., transfusion) RECURRENT/ PROGRESSIVE DIPG (STRATUM 1): Platelets >= 100,000/mm^3 (unsupported, defined as no platelet transfusion within 7 days, and recovery from post-transfusion nadir) NON-PROGRESSED DIPG (STRATUM 2): Platelets >= 100,000/ mm^3 (unsupported, defined as no platelet transfusion within 7 days, and recovery from post-transfusion nadir) Platelets >= 100 x 10^3/mm; may not be reached by transfusion Platelets >= 100,000 /uL without transfusion within 10 days prior to dose Platelets >= 100,000/ul; low platelet counts may be corrected with transfusion to achieve eligibility for study Platelets ? 100 x 109/L (in case of transfusion stable for ?14 days prior to treatment start) On a chronic transfusion program or planning on exchange transfusion during the study Has received a transfusion (platelets or red blood cells) within 3 weeks before the first dose of niraparib Has received a transfusion (platelets or red blood cells) within 3 weeks of the first dose of niraparib Platelets >= 60,000/mcL without transfusion support within the past 30 days Platelets ?50,000/mm3 independent of transfusion Platelets ?50,000/mm3 independent of transfusion Platelets > 100,000/u (transfusion independent); patients with bone marrow involvement are eligible provided they meet these criteria Platelets ? 75,000 cells/mm^3 (75 × 10^9/L), without platelet transfusion, within 72 hours of screening evaluation. Platelets >= 75,000/uL and transfusion independent, defined as not receiving a platelet transfusion for at least 5 days prior to complete blood count (CBC) documenting eligibility Platelets >= 100,000/mm^3 or >= 50,000/mm^3 if bone marrow involvement independent of transfusion support in either situation Patients with platelets >= 100,000/uL (without platelets transfusion for at least one week prior to blood test date); the most recent platelets level taken within 60 days prior to study enrollment will be considered Platelets >= 75 x 10^9/L and no platelet transfusion within the past 21 days prior to study registration Platelets >= 75,000/mcl (in the absence of platelet transfusion for >= 14 days)