This study is testing a new medicine called GS-1427 for people with ulcerative colitis (UC), a disease that causes the colon (large intestine) to become swollen and sore. The study will check if GS-1427 works well and is safe. It will compare GS-1427 to a placebo (a pill with no medicine) in Part 1, and GS-1427 with another drug called ustekinumab in Part 2. Part 1 and Part 2 both look at how well these treatments work after 12 weeks.

• Participants should have had UC for at least 90 days.
• The study takes 12 weeks, with different treatments in Part 1 and Part 2.
• Participants must have tried other UC treatments that didn’t work for them.

People who join should not have Crohn's Disease, which is another type of bowel disease, or certain other serious conditions. They also cannot have taken certain medications before. Please consider these points if you are thinking of joining the study.