This study is testing if the medicine GS-1427 helps people with **ulcerative colitis** (UC), a disease causing swelling in the colon. The study compares GS-1427 with a placebo, which looks like real medicine but has no active ingredients. The main goal is to see if GS-1427 works better than the placebo in helping people feel better by Week 12.

To join, you must have had UC symptoms for at least 90 days, with the diagnosis confirmed by a special camera test called an endoscopy. You should have a certain level of disease activity and have tried other treatments like steroids or special immune system drugs without success.

Some people cannot join, like those with Crohn's Disease, a similar but different gut problem, or those who have used certain drugs before. If you are taking certain medications that you must continue, you cannot join either.

• This study lasts for 12 weeks.
• You may need to stop some current medications to participate.
• There are risks such as possible side effects of GS-1427.