Inclusion Criteria:

          -  Diagnosis of primary of secondary myelofibrosis with transplant indication by Dynamic
             International Prognostic Scoring System (DIPSS)-plus (> intermediate-1)

          -  Patients >= age 50 must have a comorbidity score (hematopoietic cell
             transplant-comorbidity index [HCT-CI]) < 4 (Sorror)

          -  Patients can be in chronic phase (CP) with bone marrow (BM) blast count =< 15% as long
             as no evidence of disease acceleration per principal investigator (PI) and treating
             physician's opinion or after progression to acute myeloid leukemia (AML) and achieved
             =< 5% BM blasts (morphologic complete remission [CR] prior to transplant)

          -  Lack of an human leukocyte antigen (HLA) matched donor or need to proceed fast to
             transplantation when a patient does not have an immediately available matched
             unrelated donor (typed by high-resolution in the registry)

          -  Performance status >= 70% (Karnofsky); patients > 50 years should have adequate
             cognitive function; any concerns regarding cognitive function should be addressed by a
             geriatrician/neurologist

          -  Alanine aminotransferase (ALT)/aspartate aminotransferase (AST)/bilirubin =< 5 X upper
             limit of normal (ULN)

          -  Measured creatinine clearance > 60 mls/min

          -  Left ventricular ejection fraction (LVEF) >= 50%

          -  Corrected carbon monoxide diffusing capability (DLCOc) >= 50%

          -  No active infections

          -  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

          -  DONOR: Documented informed consent per local, state and federal guidelines

          -  DONOR: Genotypically haploidentical as determined by HLA typing

               -  Preferably a non-maternal HLA haploidentical relative due to data of high
                  incidence of graft failure with use of maternal HLA haploidentical cells

               -  Eligible donors include biological parents, siblings or half-siblings, children,
                  or cousins in rare instances

          -  DONOR: Absence of pre-existing donor-specific anti-HLA antibodies (DSA) in the
             recipient; Patients with pre-existing DSA could undergo desensitization per City of
             Hope (COH) standard operating procedures [SOP] and should have DSA < MFI of 2000 prior
             to conditioning at discretion of PI

          -  DONOR: Infectious disease screening performed within 30 days prior to stem cell
             mobilization per federal guidelines and is:

               -  Seronegative for HIV 1+2 antibody (Ab) and/or HIV polymerase chain reaction
                  (PCR), human T-cell leukemia virus (HTLV) I/II Ab, hepatitis B virus surface
                  antigen (HBsAg), hepatitis B virus surface antibody (HBcAb), hepatitis C virus
                  (HCV) Ab

               -  Negative rapid plasma reagin (RPR) for syphilis

          -  DONOR: Women of childbearing potential (WOCBP): Urine pregnancy testing performed
             within 7 days prior to stem cell mobilization

          -  DONOR: Is approved and completed evaluation prior to recipient initiation of the
             preparative regimen per institutional guidelines

        Exclusion Criteria:

          -  Evidence of severe portal hypertension with evidence of decompensation either with
             bleeding varices, large volume ascites, or hepatic encephalopathy

          -  In a bone marrow biopsy 4 weeks prior to start of conditioning on study:

               -  > 15% bone marrow blasts at transplant if no history of AML and per PI and
                  treating physician's opinion of disease acceleration

               -  > 5% if had previous progression to AML

          -  Human immunodeficiency virus (HIV) positive; active hepatitis B or C

          -  Patients with active infections; the PI is the final arbiter of the eligibility

          -  Patients with evidence of severe pulmonary hypertension by echocardiogram and
             confirmed by a subsequent right side cardiac catheterization pre-enrollment

          -  Liver cirrhosis

          -  Prior central nervous system (CNS) involvement by tumor cells

          -  History of another primary malignancy that has not been in remission for at least 3
             years (the following are exempt from the 3-year limit: non-melanoma skin cancer, fully
             excised melanoma in situ [stage 0], curatively treated localized prostate cancer, and
             cervical or breast carcinoma in situ on biopsy or a squamous intraepithelial lesion on
             papanicolaou [PAP] smear)

          -  Positive beta human chorionic gonadotropin (HCG) test in a woman with child bearing
             potential defined as not post-menopausal for 12 months or no previous surgical
             sterilization

          -  Noncompliance - inability or unwillingness to comply with medical recommendations
             regarding therapy or follow-up, including smoking tobacco

          -  DONOR: Has undergone solid organ, stem cell, bone marrow or blood transplantation

          -  DONOR: Receiving any investigational agents, or concurrent biological, chemotherapy,
             immunosuppression or radiation therapy

          -  DONOR: Active infection

          -  DONOR: Thrombocytopenia < 150,000 cells /mm^3 at baseline evaluation

          -  DONOR: Sero-positive for HIV-1 & 2 antibody, HTLV-I & II antibody, hepatitis B virus
             (HBV) and HCV

          -  DONOR: Medical or physical reason which makes the donor unlikely to tolerate or
             cooperate with growth factor therapy and leukapheresis

          -  DONOR: Factors which place the donor at increased risk for complications from
             leukapheresis or G-CSF therapy

          -  DONOR: WOCBP: Pregnant or =< 6 months breastfeeding