Inclusion Criteria:

• 18 years of age or older.
• Male or nonpregnant, nonlactating females.
• Diagnosis of ulcerative colitis (UC) for at least 3 months prior to screening.
• Moderately to severely active ulcerative colitis (UC) (total MCS ≥ 6), with objective evidence of inflammation defined by a Mayo endoscopic subscore (MES) ≥ 2 and disease extending > 15 cm from the anal verge.
• Physician plans to administer Janus Kinase Inhibitor Therapy for at least 8 weeks as part of standard of care (SOC).
• Documentation of a negative test result for latent tuberculosis within the last 12 months, or according to routine clinical practice.
• Able to participate fully in all aspects of this clinical trial, including collection of tissue biopsies.
• Written informed consent must be obtained and documented.

Exclusion Criteria:

• Diagnosis of Crohn’s disease or indeterminate colitis.
• An active, serious infection, including localized infections.
• Concomitant administration of biological therapies for UC or potent immunosuppressants, such as azathioprine and cyclosporine. Subjects with previous exposure to these treatments should undergo an appropriate washout period according to local practice prior to starting tofacitinib, in keeping with routine clinical practice.
• Hematology laboratory results that meet the following:
  • absolute lymphocyte count < 500 cells/mm3.
  • absolute neutrophil count < 1000 cells/mm3.
  • hemoglobin < 9 g/dL.
• Interval between live vaccinations and initiation of tofacitinib therapy should be in accordance with current vaccination guidelines regarding immunosuppressive agents.
• Serious underlying disease other than UC that in the opinion of the investigator may interfere with the subject’s ability to participate fully in the study.
• Prior enrollment in the current study and having received tofacitinib.