Inclusion Criteria:

          -  Patients with a diagnosis essential thrombocythemia (Cohort 1) or polycythemia vera
             (Cohort 2) by World Health Organization 2016 diagnostic criteria

          -  Patients with essential thrombocythemia must be very low (no history of thrombosis,
             age ≤ 60, and no JAK2 mutation), low (no history of thrombosis, age ≤ 60, presence of
             JAK2 mutation), or intermediate risk (no history of thrombosis, age >60, no JAK2
             mutation) by IPSET criteria.1 Patients with polycythemia vera must be low risk (no
             history of thrombosis and age <60) by NCCN guidelines

          -  Patients with an MPN-SAF TSS score ≥ 10 AND at least one individual feature ≥ 5
             documented on a separate visit within 3 months prior to study registration, as
             documented in the clinical record or obtained by clinician. If not previously
             documented in the electronic medical record, participants must be blinded to purpose
             of MPN SAF TSS scoring for eligibility determination. Average daily MPN-SAF TSS score
             must remain ≥10 with any individual feature ≥ 5 for the week-long baseline assessment
             prior to ruxolitinib initiation .

          -  Patients who have previously received or are receiving cytoreductive therapy (i.e.
             hydroxyurea, anagrelide, interferon) are eligible for the study if therapy was used
             for the indication of symptom control, in which case there will be a wash-out period
             of one week from prior therapy discontinuation to ruxolitinib initiation. Patients who
             temporarily required cytoreductive therapy for pre-operative control of blood counts
             prior to surgery are also eligible.

          -  Age ≥18 years.

          -  ECOG performance status ≤2 (Karnofsky ≥60%

          -  Participants must have adequate organ and marrow function as defined below:

               -  leukocytes ≥3,000/mcL

               -  absolute neutrophil count ≥1,500/mcL

               -  platelets ≥100,000/mcL

               -  total bilirubin ≤ institutional upper limit of normal (ULN)

               -  AST(SGOT)/ALT(SGPT) ≤3 × institutional ULN

               -  creatinine ≤ institutional ULN OR glomerular filtration rate (GFR) ≥60
                  mL/min/1.73 m2 unless data exists supporting safe use at lower kidney function
                  values, no lower than 30 mL/min/1.73 m2

          -  Participants with a prior or concurrent malignancy not receiving treatment for
             concurrent cancer diagnosis and/or prior concurrent malignancy within 5 years except
             for basal cell carcinoma or squamous cell carcinoma of the skin

          -  For participants with evidence of chronic hepatitis B virus (HBV) infection, the HBV
             viral load must be undetectable on suppressive therapy, if indicated.

          -  For participants with evidence of chronic human immunodeficiency virus (HIV)
             infection, they must be negative for HBV DNA, HCV RNA, or hepatitis B surface antigen
             (BsAg) on suppressive therapy, if indicated.

          -  Participants with known history or current symptoms of cardiac disease, or history of
             treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac
             function using the New York Heart Association Functional Classification. To be
             eligible for this trial, participants should be class 2B or better.

          -  Ability to understand and the willingness to sign a written informed consent document.

        Exclusion Criteria:

          -  Essential thrombocythemia patients who are high risk by IPSET-R criteria (age > 60
             with JAK2 V617F mutation and/or history of thrombosis).1 Polycythemia vera patients
             who are high risk by NCCN guidelines (age > 60 and/or history of thrombosis).

          -  Patients with >5% blasts on baseline marrow exam or at any other time in peripheral
             blood

          -  Participants who are receiving any other investigational agents.

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to ruxolitinib or excipients of ruxolitinib.

          -  Participants requiring any medications or substances that are inhibitors or inducers
             of 3A4 isozyme are ineligible. Because the lists of these agents are constantly
             changing, it is important to regularly consult a frequently-updated medical reference.
             As part of the enrollment/informed consent procedures, the participant will be
             counseled on the risk of interactions with other agents, and what to do if new
             medications need to be prescribed or if the participant is considering a new
             over-the-counter medicine or herbal product.

          -  Participants with uncontrolled intercurrent illness.

          -  Participants with inadequate liver or renal function at screening as evidenced by lab
             values not meeting criteria

          -  Participants with psychiatric illness/social situations that would limit compliance
             with study requirements.

          -  Pregnant women are excluded from this study because ruxolitinib is a Class C agent
             with the potential for teratogenic or abortifacient effects. Because there is an
             unknown but potential risk for adverse events in nursing infants secondary to
             treatment of the mother with ruxolitinib, breastfeeding should be discontinued if the
             mother is treated with ruxolitinib.

          -  The effects of ruxolitinib on the developing human fetus are unknown. Pregnant women
             and subjects of childbearing potential who are unwilling to take appropriate
             precautions to avoid becoming pregnant or fathering a child are ineligible. Women of
             child-bearing potential and men must agree to use adequate contraception (hormonal or
             barrier method of birth control; abstinence) prior to study entry and for the duration
             of study participation. Should a woman become pregnant or suspect she is pregnant
             while she or her partner is participating in this study, she should inform her
             treating physician immediately. Men treated or enrolled on this protocol must also
             agree to use adequate contraception prior to the study, for the duration of study
             participation, and 4 months after completion of ruxolitinib administration.