Inclusion Criteria:

          -  Have a breast biopsy showing ADH (atypical ductal hyperplasia), ALH (atypical lobular
             hyperplasia), LCIS (lobular carcinoma in situ), or DCIS (ductal carcinoma in situ)
             requiring surgical excision. Microinvasive disease is allowed.

               -  NOTE: Tissue from the diagnostic biopsy must be accessible/available for research
                  correlates (i.e., a tissue block or ~10 unstained slides). Due to the nature of
                  the study, fewer slides may be accepted with prior permission from the Protocol
                  Chair if there is insufficient tissue.

          -  Women and men age 18 and older.

          -  Adequate hematologic and organ function, defined as follows:

               -  Absolute neutrophil count ≥ 1500/mm3

               -  Hemoglobin ≥ 9.0 g/dL

               -  Platelet levels >200 x 109/L

               -  Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN)

               -  AST/ALT ≤ 2.5 x institutional ULN

               -  Alkaline phosphatase ≤ 5 x institutional ULN

               -  Creatinine clearance > 50 mL/min as calculated by the Cockcroft-Gault method

          -  Willing to not use concomitant strong CYP3A4 inhibitors as this could interfere with
             the metabolism of ruxolitinib (i.e azole antifungals, clarithromycin, conivaptan,
             grapefruit juice, mibefradil, nefazodone, protease inhibitors, telithromycin).

          -  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.

          -  If the patient undergoes germline genetic testing, the results must be received prior
             to randomization, as the results may affect the surgical approach and, in turn, the
             date of surgical excision.

          -  Patient understands the study regimen, its requirements, risks, and discomforts, and
             is able and willing to sign a written informed consent document.

        Exclusion Criteria:

          -  Treatment with selective estrogen receptor modulators (SERMs) or aromatase inhibitors
             for breast cancer prevention within 1 year prior to starting study treatment.

          -  Treatment with any other investigational agents within 30 days of starting study
             treatment.

          -  Current diagnosis of invasive breast cancer (current microinvasive disease is
             allowed), or previous history of invasive breast cancer diagnosed within the last 5
             years.

        NOTE: If previous history of ER+ invasive breast cancer diagnosed > 5 years ago, patient
        must be off endocrine therapy for at least 1 year prior to starting study treatment.

          -  Uncontrolled intercurrent illness including, but not limited to ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, end stage renal disease (ESRD), or psychiatric illness/social situations
             that would limit compliance with study requirements.

          -  Women who are pregnant or nursing.

          -  HIV-positive participants on combination antiretroviral therapy are ineligible because
             of the potential for pharmacokinetic interactions with ruxolitinib.

          -  Prior or current treatment with a JAK inhibitor, for any indication.

          -  Known active Hepatitis B or C.