Received JAK inhibitor therapy after allo-HSCT for any indication. Treatment with a JAK inhibitor before allo-HSCT is permitted. CRLF2 rearrangement with JAK1 or JAK2 mutation (JAK+) CRLF2 rearrangement without JAK mutation Other JAK pathway alterations (eg, JAK2 fusions, erythropoietin receptor (EPO-R) fusions, SH2B3 deletions, interleukin-7 receptor-alpha (IL7RA) mutations) with or without CRLF2 rearrangement Patients who have previously been treated with an IL-6, JAK or STAT inhibitor for any indication, such as ruxolitinib or tocilizumab Participants who were treated with prior JAK inhibitors for aGvHD; except when the participant achieved complete or partial response and has been off JAK inhibitor treatment for at least 8 weeks prior to Cycle 1 Day 1 Patients previously treated with a Janus kinase (JAK) inhibitor Hypersensitivity to any JAK inhibitor Prior treatment with JAK2 inhibitor therapy is not excluded. Patients on a JAK2 inhibitor may continue through conditioning until day -3 then tapered at the discretion of the investigator. Patients must not have received prior JAK1 inhibitor therapy. Prior treatment with JAK2 inhibitor therapy is not excluded; a JAK2 inhibitor will need to be stopped 1-2 days prior to starting conditioning regimen Subjects who have previously received JAK inhibitor therapy Treatment with ruxolitinib or other JAK inhibitor in the past Any previous use of Janus kinase (JAK) inhibitor, osimertinib, or other EGFR-directed therapy for T790M-mt NSCLC. PART 2: Received JAK inhibitor for at least 8 weeks immediately prior to conditioning and be able to continue until Day -4 pre-transplant Patients cannot have any other form of chemotherapy for their MPN (other than hydroxyurea); specifically prior interferon or JAK2 inhibitors are prohibited Any previous Janus kinase 2 gene (JAK2) inhibitor treatment prior to study enrollment, with the exception of ruxolitinib Hypersensitivity to Janus kinase (JAK) inhibitor Documented progressive disease during or after JAK inhibitor therapy MF PATIENTS: Patients must be intermediate I risk or beyond and meet the following:\r\n* In need of treatment\r\n* Intolerant or refractory to ruxolitinib (or other investigational Janus kinase [JAK]-inhibitors) OR unlikely to benefit from ruxolitinib\r\n* Ineligible or refusal to undergo stem cell transplantation Any prior or concomitant use of another JAK inhibitor Previously received JAK inhibitor therapy for any indication. Prior use of a JAK1 or JAK2 inhibitor Patients must be resistant to, intolerant of, or ineligible for Janus kinase (JAK)2 inhibitor therapy, or have refused such therapy History of prior janus kinase (JAK) or STAT3 inhibitor treatment Previous allergic reactions to janus kinase (JAK) inhibitors or excipients Prior therapy with ruxolitinib or other JAK inhibitors Any chemotherapy (e.g., hydroxyurea), immunomodulatory drug therapy (e.g., thalidomide), immunosuppressive therapy, corticosteroids > 10 mg/day prednisone or equivalent, growth factor treatment (e.g., erythropoietin) or Janus kinase (JAK) inhibitor therapy =< 14 days prior to registration Any prior therapy with JAK2-tyrosine kinase inhibitor (TKI), hypomethylating agents, histone deacetylase inhibitor (HDACI), mechanistic target of rapamycin inhibitor (mTORi), or immunomodulatory drugs (iMiDs) is allowed as long as it is greater than 3 weeks since last dose of administration and in the case of a JAK2-TKI or HDACI that discontinuation was not due to non-hematologic drug toxicity; an exception to this criteria are patients currently on at least 10mg BID of ruxolitinib for greater than 3 months and who have not shown an optimal response (i.e. without 50% reduction in palpable splenomegaly or 50% reduction in symptom burden); with a reduction of ruxolitinib to 10mg BID these patients may enter onto the study without stopping ruxolitinib MF patients must have received prior JAK2 inhibitor therapy, and been found to be intolerant, or refractory/relapsed from prior JAK2 inhibitor therapy, based on investigator assessment Subjects previously treated with anagrelide or JAK inhibitors. Prior treatment with a JAK inhibitor for any indication. Subjects with any prior exposure to Janus kinase 2 (JAK2) inhibitor therapy (i.e. ruxolitinib or prior pacritinib therapy) are excluded Prior treatment with a JAK inhibitor for any indication. Prior treatment with more than 2 JAK2 inhibitors or with pacritinib There is no maximum cumulative prior JAK2 inhibitor treatment Treatment with a Janus kinase (JAK) inhibitor within 21 days of the planned first dose of MMB Hypersensitivity to Janus kinase (JAK) inhibitor Prior therapy with a janus kinase 2 (JAK2) or fms-related tyrosine kinase 3 (FLT3) inhibitor Prior therapy with a JAK inhibitor (other than ruxolitinib for less than 3 months duration and currently on it) or histone deacetylase inhibitor (HDACi); patients that are currently on ruxolitinib for less than 3 months of therapy are eligible Patients who have received prior treatment with JAK inhibitor Patients must be off myeloproliferative neoplasm (MPN) directed therapy, such as Janus kinase (JAK)-inhibitors, for at least 2 weeks prior to administration of the study drug; NOTE: This does not include supportive transfusion, or hydroxyurea; these must be stopped prior to first day of treatment, but no wash –out period is required Patients must be resistant to, intolerant of, or ineligible for JAK2 inhibitor therapy, based on severe anemia or thrombocytopenia Prior treatment with more than two Janus kinase 2 (JAK2) inhibitors or with pacritinib Moderate or severe cGVHD diagnosed and staged per National Institute of Health (NIH) criteria; responses to Janus kinase (JAK) inhibitors have not been restricted to specific organs, so any organ involvement is eligible Hypersensitivity to JAK inhibitors Subjects who have previously received JAK inhibitor therapy Part B only: Prior treatment with SYK or Janus Kinase (JAK) inhibitors, except MF subjects. Prior treatment with a JAK inhibitor or with an investigational agent, device, or procedure within 21 days of enrollment. Myeloproliferative neoplasms (MPN): must have < 5% peripheral / marrow blasts\r\n* Note: Prior use of a Janus kinase 2 (JAK2) inhibitor is allowed up to 4 weeks before day 0 of allogeneic hematopoietic cell transplantation (alloHCT) Any patient anticipating or scheduled to receive a tyrosine kinase inhibitor, FLT3 inhibitor, or JAK2 inhibitor (outside of this study) post-HCT