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.
