Ruxolitinib in Seborrheic Dermatitis

Inclusion Criteria For SD Subjects: * Male or female subjects ≥ 18 years of age at the time of signing the informed consent document. * Subject is able to understand and voluntarily sign an informed consent document prior to participation in any study assessments or procedures. * Subject is able to adhere to the study visit schedule and other protocol requirements. * Baseline SD score of IGA ≥ 3 with facial involvement * Subject agrees to discontinue all treatments for SD from screening through study completion aside from the study drug * Subject has failed an adequate course of treatment with at least one available therapy (topical antifungals or low-potency topical corticosteroids) * Subject is judged to be in otherwise good overall health as judged by the investigator, based on medical history, physical examination, and laboratory testing. (NOTE: The definition of good health means a subject does not have uncontrolled significant co-morbid conditions). * Females of childbearing potential (FCBP) must have a negative pregnancy test at Screening and Baseline. While on the study drug and for at least 90 days after the last application of the study drug, male and female participants must be willing to take appropriate contraceptive measures to avoid pregnancy or fathering a child. FCBP who engage in activity in which conception is possible must use one of the approved contraceptive options described below: * Option 1: Any one of the following highly effective contraceptive methods: hormonal contraception (oral, injection, implant, transdermal patch, vaginal ring); intrauterine device (IUD); tubal ligation; or partner's vasectomy, OR: * Option 2: Male or female condom (latex condom or nonlatex condom NOT made out of natural \[animal\] membrane \[for example, polyurethane\]); PLUS one additional barrier method: (a) diaphragm with spermicide; (b) cervical cap with spermicide; or (c) contraceptive sponge with spermicide. The female subject's chosen form of contraception must be effective by the time the female subject is enrolled into the study. Inclusion Criteria For Control Subjects: * Male or female subjects ≥ 18 years of age at the time of signing the informed consent document. * Subject is able to understand and voluntarily sign an informed consent document prior to participation in any study assessments or procedures. * Subject does not currently have and does not have a history of SD. * Female of childbearing potential (FCBP) must have a negative pregnancy test at Screening and Baseline Exclusion Criteria For SD Subjects: The presence of any of the following will exclude a subject from enrollment: * SD clinical severity of IGA \<3 and SD Severity Score \<6. * Subjects with other skin diseases that would interfere with the study assessment in the opinion of the investigator. * Active bacterial, fungal, or viral skin infection within 2 weeks from study initiation. * Subject has clinically significant (as determined by the investigator) renal, hepatic, hematologic, intestinal, endocrine, pulmonary, cardiovascular, neurological, psychiatric, immunologic, or other major uncontrolled diseases (e.g., malignancy, TB, HIV, HBV, HCV, thromboembolic events) that will affect the health of the subject during the study, or interfere with the interpretation of study results. * Subject has previously received treatment with oral or topical JAK inhibitors * Current other topical treatments (e.g., topical corticosteroids, topical calcineurin inhibitors) within 1 week of baseline * Use of systemic immunosuppressive medications, including, but not limited to, cyclosporine, systemic or intralesional corticosteroids, mycophenolate mofetil, azathioprine, methotrexate, tacrolimus within 4 weeks of study initiation * Concurrent use of strong CYP3A4 inhibitors within 7 days or 5 half-lives (whichever is longer). A list of CYP3A4 inhibiting medications can be found in Appendix 3. * History of adverse systemic or allergic reactions to any component of the study drug. * Current participation in any other study with an investigational medication * Subject who is pregnant or breast feeding Exclusion Criteria For Control Subjects: * Active bacterial, fungal, or viral skin infection within 2 weeks from Screening/Baseline visit. * Subject has uncontrolled clinically significant (as determined by the investigator) renal, hepatic, hematologic, intestinal, endocrine, pulmonary, cardiovascular, neurological, psychiatric, immunologic, or other disease. * Subject has previously received treatment with oral or topical JAK inhibitors * Current other topical treatments (e.g., topical corticosteroids, topical calcineurin inhibitors) within 1 week of baseline * Use of systemic immunosuppressive medications, including, but not limited to, cyclosporine, systemic or intralesional corticosteroids, mycophenolate mofetil, azathioprine, methotrexate, tacrolimus within 4 weeks of study initiation * Current participation in any other study with an investigational medication * Subject who is pregnant or breast feeding