The purpose of this observational study is to learn about the effects of etrasimod as a treatment for adult patients with moderate to severe ulcerative colitis. Patients will be treated according to standard of care and will only be included in the study if etrasimod is the best treatment of choice according to the physician and they have not previously taken etrasimod.

All patients will be prescribed etrasimod according to standard of care. Tests and doctor visits will be conducted according to standard of care with the exception of health questionnaires about ulcerative colitis symptoms. These questionnaires will be completed by patients at various timepoints during the study using their mobile phone, tablet, or computer.

The study is 52 weeks with 28 days of safety follow up. The effects of etrasimod will be analyzed for each patient comparing their disease activity prior to the start of etrasimod.

Inclusion Criteria:

1. Age ≥ 18 years and < 65 years at baseline
2. Patients with confirmed diagnosis of UC who are prescribed etrasimod for moderately to severely active UC
3. Evidence of a personally signed and dated ICD indicating that the patient (or a legally acceptable representative)has been informed of all pertinent aspects of the study

Exclusion Criteria:

1. The presence of clinical findings suggestive of Crohn's disease

2. Severe extensive colitis evidenced by:

   1. Physician judgement that the patient is likely to require hospitalization for medical or surgical intervention of any kind for UC (e.g., colectomy) within 12 weeks
   2. Current evidence of acute severe UC, fulminant colitis, or toxic megacolon

3. Patients with a stoma or planned UC surgical intervention requiring hospitalization

4. Prior/Concomitant Therapy:

   1. Any previous exposure to etrasimod, including participation in the etrasimod clinical program
   2. Any co-medication with one of the following conventional therapies: methotrexate or a thiopurine (azathioprine, mercaptopurine, or tioguanine)
   3. Any co-medication with one of the following advanced therapies for UC: biologics (a TNF, integrin or cytokine antagonist; JAKi [filgotinib, tofacitinib, or upadacitinib]) or with any other S1P receptor modulator

5. Unwillingness or inability to download the web-based tool to complete ePROs on a personal device or not capable of using the web-based tool

6. Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family

Eligible patients can be included in this study if their physician makes a clinical decision to treat them with etrasimod, as defined by the United States etrasimod prescribing information and independent from the decision to enroll the patient in this study.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.